You do not need a licence to work on systems below 50 volts ac, nor below 110 volts dc. You must however be ultra-careful never to allow battery positive and negative leads to touch – even momentarily – as the resultant current flow can literally vapourise a spanner. Always remove rings or jewellery, and wear safety glasses if working on or near the battery or wiring.
Always disconnect battery cables prior to working on the electrical system. Remove the negative cable first. To reconnect, connect the positive cable first, to avoid the spanner touching earthed metal, and creating a hugely hot arc. Of completed work, the main risk is of excess current melting cable insulation and igniting flammable material. Guard against it by installing circuit breakers at the battery end of cables, and fuses at the appliance end.
Have any 230 volt wiring installed by a licensed electrician experienced in RV work. This applies even if there is no provision for grid connection. The electrician can advise on the location of switches and safety devices. He or she must do all the 230 volt wiring and connections and certify the RV’s electrics for registration – following the requirements of AS/NZS 3001:2008 (as Amended in 2012) plus those of AS/NZS 3000:2007 (as Amended in 2012).
Free-standing inverters may be self-installed. Appliances must be plugged in directly (or via a multiple outlet adaptor or board). Pic: Jaycar
It is legal to install an inverter yourself only if it is a ‘stand-alone’ type that has inbuilt power outlets into which you plug appliances directly. These outlets must not be connected to fixed 230 volt wiring.
A typical dc circuit has two conductors. That from battery positive runs to a nearby main dc circuit breaker (it can double as the main switch). It is good practice to connect the output side of that switch to a power post – or multiple connector box. Cables from that post or box run to the positive of the various loads, ideally via (lower current) circuit breakers. A second (negative) conductor runs directly from battery negative to a second common power post, or multiple connector box, to the negative side of each load.
Fuses are cheaper than circuit breakers, but can be poor economy. Many fuse holders are poorly made and later cause problems. Dividing the wiring into sections enables circuit breakers to be used to readily switch or isolate specific areas of the system.
The metal chassis earth return is commonly used by vehicle builders for the obligatory on-road lights. That must be left as is. Earth return is best avoided for the auxiliary electrics. This is because it is difficult to ensure long-term electrical soundness. As any auto electrician will confirm, many electrical faults are traced to poorly made and/or corroded earth connections.
Earth return wiring: the chassis doubles as the negative lead. Drawing: RV Books.
Recommended wiring practice for trailers and added bits in the towing vehicle. Note that if the battery and fridge are 3 metres apart, the conductor length is 6 metres
Cables may be single, or have positive and negative leads in one common sheath. Unless otherwise stated, cable sizes in this book refer to total conductor length. There are usually negative and positive conductors so the overall length for each circuit is thus twice the physical distance.
This also applies to twin cables (i.e. where two conductors are within the one single sheath). This may seem obvious but is often misunderstood.
Of the nominal twelve volts of a lead acid battery, only that between about 11.6-12.8 volts is usable. It is thus essential to limit voltage drop between battery and load.
Trailer makers and installers generally regard a drop of 0.35-0.50 volts as acceptable, but that effectively renders some 30% of battery capacity inaccessible. In practice, all systems work far better if that drop is kept to 0.1-0.15 volt. This is readily achievable in camper trailer wiring and assists reduce losses with dc-dc charging (how to do this is shown on following pages). LifePO4 batteries maintain 13.1-12.9 volts under any realistic camper trailer load. Despite this, still use cabling as shown below and on page 57.
Run cabling by the shortest route, not the easiest. If that is not feasible, increase its size accordingly. Excess voltage drop can also be introduced by loose or poorly-made connections in plugs, sockets and switches, and by cheap fuse holders as they age and corrode.
The auto cable trap
The most common cause of (accidental) major voltage drop is through using ‘auto cable’. This is what you are invariably sold if buying from auto parts and hardware stores even if you ask specifically for the type and size actually needed. It is a major trap that results in countless RV wiring issues.
When an appliance maker specifies (say) 4.0 mm² cable, that means exactly what its says: i.e. use cable that has a conductor cross-sectional area of 4.0 mm². Auto cable however is rated as its overall diameter (insulation and all). As insulation thickness varies from brand to brand, an auto cable’s rating indicates only the size hole you can just poke it through.
Worse, of the most commonly specified sizes, auto cable has the same ‘numbers’ as those in mm² (i.e the auto cable ‘numbers’ are the same but what they measure is much smaller). In the most common smaller sizes that difference is huge, e.g. one of the most commonly-specified cable sizes is 4.0 mm² – there is also a 4.0 mm auto cable, but rather than being 4.0 mm², its conductor is only 1.8-2.0 mm² – thus doubling voltage drop. Smaller sizes are even worse. If this is not understood, the auto-cable you are sold will be 25%-75% too small!
The incorrect use of auto cable sizes is the cause of most 12 volt issues when work is self-done and even some professionally.
USA, Canada and a few other countries use AWG, or the almost identical B&S standard. The rest of the world uses ISO (International Standards Organisation) metric ratings – logically based on the copper conductor’s cross-sectional area in mm².
As auto cable conductor sizes vary from maker to maker accurate conversion is not possible. The Table below shows only that probable. Very few staff in auto parts stores are aware of this situation (I have yet to meet one). So even if you ask specifically for (say) 4.0 mm² cable you will inevitably be sold 4 mm auto cable (that is only 1.8 -2.0 mm²). The size is, however, usually shown on the drum, and in the makers’ technical literature that the store is likely to have.
The problems that confusion over auto-cable may last for the life of the RV. It is why many RV fridges do not work that well, and/or draw excess current on hot days – they never get cold enough to reach their shut-off temperature.
Relationship between cables. Auto cable comparison is approximate (conductor sizes vary from maker to maker.) Exact equivalents are not possible. Few vendors stock ‘odd number’ AWG/B&S sizes. For 4.0 mm² (approx AWG 11) use the heavier AWG 10 instead. Table: RV Books.
Knowing what size cable to use
The preferred way by far is via the following minor sum.
Where L is the conductor length, and I is the current (in amps), the voltage drop is:
L x I x 0.017 divided by the conductor’s cross sectional area (in mm²).
Example: For a 5 metre length of dual cable carrying 5 amps, the voltage drop is: 10 (m) x 5 (A) x 0.017 = 0.85 – divided by 4.0 (mm²) = 0.21 V. That’s a little too high, but 6 mm² results in a comfortable 0.14 volt drop.
Note: Purists use 0.0164 (not 0.017) because it is an ISO standard constant – but 0.017 is very close and easier to remember. The Table below is for those who prefer not to do sums. It shows approximate current and cable lengths only. It is calculated for a maximum 3% voltage drop.
Cable is also sold by ‘current rating’, but all that ‘rating’ indicates is the current it can carry before its insulation is at risk of melting. Twin 4.0 mm auto cable (about 1.8-2.0 mm²) can carry 12 amps over one metre (i.e. 2 metres of conductor) with 0.2 volt drop, yet has a voltage drop over six metres (12 metres of conductor) of 1.22 volt. Yet that cable may be ‘current-rated’ at 20-50 amps – with no mention of distance.
A typical shopping scenario: ‘I need three metres of twin cable for this 12 volt electric jug – it draws 13 amps’. ‘Sure mate, this 2 mm stuff is current-rated at 50 amps.’ But three metres of twin 2 mm auto cable carrying 13 amps drops 0.66 volts. Fine over a couple of metres, but not more.
High quality auto cable is fine as such. It is readily available and affordable but the way it is rated is electrical madness. If you intend to use it, you must know its conductor area or, at least use the approximations shown on page 56.
Never use the single (or seven strand) copper wire used in houses for RVs. It work-hardens and is likely eventually to fracture as a result of vibration when travelling.
Ideally, use the ISO size rated ‘tinned copper’ cable used in boat wiring. It lasts for decades and will not corrode. It is available from some boating suppliers and also from Springer Low Voltage Electrics in Brisbane.
There is no overall colour convention. Britain, Australia and New Zealand traditionally use red for positive, and black or (sometimes yellow) for negative. Jayco Australia however uses black for positive and white for negative. USA and Canada mostly do the same. Assume nothing!
On electrical appliances, diagrams etc., positive is indicated by a plus (+) sign, negative by a minus (-) sign, or shown as +ve, and -ve. Positive leads go to positive terminals. Negative leads go to negative terminals.
length <3 m
length <4 m
length <6 m
length <8 m
length <10 m
|3.0 amps||1.5 mm²||1.5 mm²||1.5 mm²||1.5 mm²||1.5 mm²|
|5.0 amps||1.5 mm²||1.5 mm²||1.5 mm²||1.5 mm²||2.5 mm²|
|7.5 amps||1.5 mm²||1.5 mm²||2.5 mm²||4.0 mm²||4.0 mm²|
|10 amps||2.5 mm²||2.5 mm²||4.0 mm²||4.0 mm²||6.0 mm²|
|15 amps||2.5 mm²||4.0 mm²||6.0 mm²||6.0 mm²||8.0 mm²|
|20 amps||4.0 mm²||4.0 mm²||6.0 mm²||10 mm²||10 mm²|
|30 amps||6.0 mm²||6.0 mm²||10 mm²||16 mm²||16 mm²|
|50 amps||10 mm²||10 mm²||16 mm²||25 mm²||25 mm²|
This cable industry table gives approximate sizes for varying situations. It is included only for those who prefer not to do the calculations described in the main text above.
Making connections (general)
Cables connect to electrical devices via plugs and sockets, crimp lugs, or by set screws. The lugs have a tubular section into which (if done correctly) the crimping action forms a ‘cold weld’.
Crimp lugs and connectors are made in various forms as well as sizes. Pic: RV Books.
To ensure this is possible it is essential to use high quality crimp lugs of extruded tubular construction. High quality crimp lugs are stocked by electrical wholesalers. The best ones work very well (some are aviation-rated). The poor ones, however inevitably cause issues after a year or three. Some cheap crimp lugs have a seam (under the plastic sleeve) that opens over time.
Small crimp lugs are colour-coded: red is used for 1.0-1.5 mm² cable, blue for 1.8-2.5 mm² and yellow for 4-6 mm². It is essential to use the right size.
Larger crimp lugs are available but are not colour coded.
Never use pliers for crimping. A ratchet operation crimping tool (about $100) is vital to provide the extreme pressure needed to form that ‘cold weld’.
Battery and inverter lugs need crimping by a hydraulic tool. It is impossible to do this yourself, but auto-electricians will do it for you, usually at nominal cost.
A ratchet-operating crimping tool is essential. Pic: Elpress.
Soldering is seriously not advisable. It locally stiffens the cable. Subsequent movement, including vibration, may cause it to fracture. Also, acid flux works it way into the copper strands. This corrodes the copper, and eventually the crimp lug itself.
Some crimp connectors are now available with pre-fitted sleeves that, after crimping, are heated and shrink to provide strength and waterproof protection.
Use plastic ties to support cables against movement.
These ties are available in 50-100 unit packs from electrical wholesalers, and are often cheaper and a lot stronger than those from hardware stores. Wholesalers sell small quantities as long as you pay cash or by credit card.
Joining cables etc.
Most trailer and caravan batteries have multiple cables attached with varying effectiveness and probability of working loose. Instead, run a single heavy cable from each main battery terminal to a nearby common terminal post or connection box.
This power post was made from scrap material by the author. Pic: RV Books.
These are available commercially, or readily made from scrap material – as shown (right).
To join single cables, use the small insulated connectors made for three-core mains wiring (available from electrical wholesalers).
Connectors such as this are made in a large range of configurations and cable sizes. Pic. RV Books.
Multiple cables can be joined via the larger connector boxes made for linking multiple active and neutral cables in 230 volt circuits. They are available in various sizes and accommodate from three to 20 or more separate cables. Red, black and grey tinted outer cases assist identifying polarity. Extra heavy cables are best joined via terminal posts.
Be wary of the ‘Scotch connectors’ used to tap into existing wiring. They are a small metal version of a piranha. They bite through existing cable and into the copper conductor. These connectors may be fine for a quick fix, but a junction box is a more permanently satisfactory alternative, particularly in exposed areas.
Circuit breakers and fuses
DC circuit breaker.
One of the more serious electrical faults is live conductors accidentally touching or appliances overheating. Both may cause such heavy current to flow that cables melt and catch fire, igniting anything flammable nearby. Circuit breakers and fuses protect against such faults.
Circuit breakers, inserted at the battery end of cables, protect them from overheating if current flow exceeds cable capacity. Most double as switches.
Whilst more costly, use only circuit breakers made specifically for dc (Clipsal has a good range). Blade fuses are a fraction of the price, but insurance records show they do not protect as well.
Fuses, at the appliance end of cables, protect against further damage if an appliance fault causes excess current to flow.
Plugs and sockets
Few cigarette lighter plugs and sockets have mechanical locking. Some sooner or later work loose and arc internally. They are a known cause of fire, and of voltage drop.
Hella cigarette lighter plug can carry 8 amps. Pic: Hella.
The Hella unit however does have mechanical locking. It is rated to carry 8 amps at 12 volts. Part numbers are 4918 (socket) and 4952 (plug) but they are not stocked by all auto-parts stores. Bulgin too makes a superb marine plug and socket.
Batteries are usually drawbar mounted: if so house them in battery boxes. They are, however better low down and ahead of (or very close to) the axle. They must be securely bolted down as they are damaged internally if they rattle around. Ventilation is essential (see page 29). If having more than one auxiliary battery, consider locating one in the towing vehicle and the other one in the trailer.
Ensuring safe starting
The auxiliary battery is, in most systems, charged via the alternator. It is effectively in parallel with the starter battery. As it is vital to be able to restart the engine, the starter battery is isolated from the auxiliary battery unless the engine is running and the alternator effectively charging.
As explained on page 32, pre-2013 vehicles do this via a voltage sensing relay, e.g. from Ingram, InterVolt, Redarc etc. These are heavy current switches that prevent the auxiliary battery (and loads) being connected until the starter battery current is replaced. This usually requires only one to two minutes. The relay also disconnects the auxiliary system if or when the starter battery falls below 12.6 volts. This is no longer feasible with variable alternators. Pages 31-33 refer.
Solar modules may be mounted on a rack on top of the trailer and angled in camp to provide shade. Alternatively, modules may be carried loose, mounted on the towing vehicle, or on both vehicles.
Unless you mount them so that they double as a sun shade, it is no longer worth attempting to tilt modules to face directly into the sun. It is now much cheaper and simpler to locate them horizontally and add 15%-20% more solar to compensate. Heat-affected solar modules (which is all except those using amorphous technology) need a 25-50 mm air gap for heat to escape.
Most modules have strong alloy frames. There are various ways of attaching these frames to the roof, depending largely on whether the underlying material is able to take screws or bolts; and whether any warranty is invalidated by drilling holes.
Sikaflex 252 is often used but as it sticks instantly it is vital to triple-check you’ve placed things correctly and that you can access nuts and bolts to remove the modules.
Check that the adhesive is compatible. Adhesives containing petroleum and silicon affect some rubber-based materials.
If adhering modules, ensure the surface is solid – not a veneer or a just a coat of paint.
Flexible stick-on solar modules cost a lot more and can cope with even steeply curving surfaces, but there have been ongoing reports of early failures.
For earlier vehicles with alternators that produced plus 14 or so volts, voltage-sensing unit is either connected to a main solenoid (as shown) or the two functions are combined in one unit. Pic: Redarc Australia.
Connecting solar modules
Any number of similar voltage solar modules can be parallel connected. They can, however have different wattage outputs (wattage is additive).
Connections are via rear-mounted boxes. You usually need to complete the wiring before securing the modules in place. If feasible, locate the modules so their connection boxes are adjacent. This reduces cable lengths.
Take the feed to the batteries and solar regulator via the shortest practical cable run – or use heavier cable – as excess voltage drop is overcome by using heavier cable. It just costs more.
Rigid solar modules need mounting like this – they need a 20-50 mm air gap – enabling them to provide good heat insulation. Pic: RV Books.
Cables may be taken into the trailer via a gland such as Whitworth Marine’s Cat No. 33516. It costs about $40 but withstands ocean waves. Or taken via a conduit box -sold by electrical wholesalers.
Connecting solar regulators
Low cost solar regulators control charging by varying the effective input – usually via high speed switching of feed between the solar modules and the battery/s.
Solar regulators connections vary, but most are more or less like this Plasmatronic unit. Follow the maker’s instructions to the letter. Many faults are caused by omitting seemingly unnecessary but vital voltage sensing cables. Pic. RV Books.
The readout needs to be readily visible but consistent with keeping supply cables as short as feasible – or oversized if that is not feasible. Some cooling air flow is required.
The drawing (right) shows connections for a negative lead switching unit.
Positive lead switching units take the solar negative directly to battery negative.
Other types may be connected differently. Most, for example, have a separate twin cable for monitoring. Some installers omit this (to save a probable under $1 cable) resulting in flawed voltage references. If that happens, the regulator will never charge correctly as that flawed reference may cause the batteries to appear more charged than they are. As a result the charge rate is prematurely terminated- and batteries wrecked.
A contact breaker or fuse in the battery end of the positive cable from the battery to the solar regulator protects against a short circuit along its length. It needs to be at the battery end.
There is no need to fuse solar modules. They withstand even a permanent and total short circuit without harm.
Loose solar modules require heavy connecting cable. Cable/s can be connected by one or more Anderson plugs and sockets. It is overkill, but there’s little else around that can be relied upon.
Parallel charging sources
This puzzles many people. In practice batteries and electrical loads draw current from whatever source has the higher voltage input at any time. It does not follow that a greater charge will be obtained via parallel connection from sources with slightly different voltages.
Solar modules etc. are electrically unidirectional. There is thus little risk of one source discharging through another.
For load monitoring many regulators require all negative load leads to connect to Load -ve (as shown here), others may require all positive load leads to go to Load +ve. Always follow the maker’s instructions. Pic RV Books.
For solar regulators to show what’s happening, some require negatives from appliances to connect to the regulator’s Load -ve. Others may require appliance positives to be connected to Load +ve.
A generally better way is to use a current shunt – as described below. This enables you to monitor any load that the shunt can carry (typically 150-300 amps) and also the alternator input.
Stand-alone energy monitors, can be located wherever they are most convenient to read. Cable length and voltage drop is rarely an issue.
Monitoring current heavier than the regulator can handle requires a ‘current shunt’. This causes a small and current-proportional voltage (typically 0.1 volt per 100 amps) to be produced across it when current flows. That voltage is registered on a meter that is calibrated to read in amps.
The shunt is wired (in series) in the main feed to/from the auxiliary battery and preferably close to that battery. See the solar regulator maker’s instructions re insertion in the positive or negative main battery cable. A light cable connects the shunt to the energy monitor.
To ensure compatibility buy the shunt from the solar regulator’s maker – or check to ensure it is correct.
Current shunt. Pic: RV Books.
With shunt operation, the Load terminal on the regulator is not used for load measurement.
All cables previously so connected go to the non-battery side of the shunt. (In some regulators, that Load terminal can be then programmed for various switching functions.)
Some solar regulators (e.g. the Plasmatronic PL series) that record incoming solar current internally, must, if a shunt is used, have the solar feed taken directly to the battery. If routed via the shunt, that the readout will show solar input as twice of that which it really is.
Shunt connection is a bit tricky. Unless you are sure of what you are doing, installing it is better left to an auto-electrician.
Current shunt is located close to the auxiliary battery. A thin twisted lead is connect to the solar regulator or energy monitor. Pic: RV Books.
Locate the inverter close to the battery. Connect it via a cable that can carry the inverter’s maximum output at low voltage drop. Include a high current fusible link in the positive lead to the inverter and as close to the battery as possible. That link should be rated at about 130% of the inverter’s peak current. A circuit breaker is preferable, but ones that size must be dc-rated.
Battery and inverter cables must be protected against breaking, working loose or being damaged in any way. Have the cables made up by an auto electrician as they are far too heavy to crimp by hand.
Inverters may be badly damaged by reversed polarity (i.e. positive and negative accidentally reversed). Not all have inbuilt wrong polarity protection. Triple check before connecting the power.
Locate the battery charger in a separate compartment from the batteries – even if those batteries are sealed. If you wish you can parallel batteries across most charging sources (such as the alternator and solar). It is not necessary to switch these sources. They have diodes or suchlike that prevent one feeding back into the other. Here too, the battery will draw most charge from whatever has the higher voltage at the time.
Changing halogen lighting or conventional globes to LED results in similar light levels at a fraction of the previous energy draw. Further, cable that was previously too thin will be fine with the very low current drawn by LEDs.
Consider installing a small outside LED light that can be switched from the bed. Night-time bumps and scratches are usually possums seeking left out Tim Tams, but it is comforting to know for sure.
The still used compact fluorescent globes are fairly efficient and can be run directly from 230 volt ac via grid power, or via an inverter from 12 volts dc. If you minimise energy usage it is better to replace them with by 12 volt LEDs because whilst even where the globes themselves to be efficient, if they were the only 230 volt load switched on at the time, the inverter’s internal losses may double the lighting system’s energy draw.
Water resists being pumped through small diameter hose and tight bends. Use at least 13 mm (ideally 19 mm) and sweeping curves rather than right angles. Use food-rated hose for pumping drinking water, and stainless steel hose clips.
Twelve volt pumps draw 4-7 amps whilst running and twice that whilst starting. Use appropriately heavy cable for the starting load as pumps can stall and overheat if the voltage is too low.
Pic: RV Books.
Install a good quality blade-type fuse close to the pump. Try a 10 amp slow-blow type first. If this blows frequently, replace it by a 15 amp normal type fuse.
Most earlier such pumps have an internal pressure switch that turns the pump rapidly on and off to maintain pressure. If a pump pressure switch fails it is often possible to bypass it and install an external one as shown here. Irrigation suppliers sell them – they cost a fraction of pump prices.
Constant pressure pumps
In recent times, some pumps maintain constant pressure. They run at more or less constant speed, pumping excess pressurised water around a loop within the pump. They are not totally silent, but are quieter than constantly cycling pumps.
Quietening water pumps
Older water pumps in particular can be disturbingly noisy whilst running, and particularly prone to ongoing ‘chirping’ all night due to the pressure regulator attempting to compensate for pressure changes as the piping expands or contracts due to temperature changes.
Typical pressure tank (exploded view). Some are supplied with the pump.
Other noise is transmitted to the RV’s structure both via the pump itself and also via associated piping.
To reduce that noise, firstly secure the pump in a manner that whatever it is bolted to does not form a sounding board. Do not overtighten its rubber mountings – it should be free to move slightly as it operates.
Connect it to the tank via a loosely held loop of flexible hose – and likewise for the pump to the RV’s water piping. Have all piping to taps etc. loosely held in place – it should be free to move slightly.
Also and very effective for quietening is to include a so-called ‘pressure accumulator’. This is a pressure vessel sold by irrigation suppliers. It is a usually cylindrical tank that contains an inflatable balloon that is inflated to about 210 kPA (30 psi). The water pump’s pressure regulator typically switches the pump on when it detects pressure below that. It then turns the pump on until pressure reaches about 300 kPA (about 40 psi), compressing the balloon to about half its volume. The pump then turns off. From then-on, when a tap is turned on, water is pumped silently via air pressure alone. Once the tank pressure is down to about 210 kPa (30 psi) – which may be many hours later – the cycle recommences.
Whilst the pipe pressure still changes it is so gradual that few people even notice it. Unless there is a dripping tap – even a 10 litre tank ensures a good night’s sleep. When the pump runs it does so for only the minute or so required to restore pressure.
Three-way fridges of 60-120 litres draw 10-12.5 amps. They use 12 volts only whilst driving, and during short rest stops, and LP gas or 230 volts at all other times. Their draw is too high to run from solar, or a camper trailer battery except for lunch stops etc.
How a pressure tank is incorporated in the system. The tank can be located anywhere that is convenient – it can be metres away from the pump if so desired. Pic: RV Books.
If running from alternator power from most pre-2008 or so vehicles, you can run them via very heavy cable from the alternator voltage sensitive relay to the fridge and battery charger, but this is always a compromise.
A better way, but still necessitating heavy cable, is the previously discussed dc-dc alternator charger located close to the trailer battery and associated fridge. This will provide full battery voltage at the fridge with only minor energy loss.
Electric (only) fridges draw 1.5-5.0 amps. All are affected by voltage drop and if within two to three conductor metres from the battery need 6.0 mm² cable.
Fridges – physical installation
A fridge is simply a pump inside a box that moves heat from where it is not wanted to where it does not matter. Correct installation creates huge differences in cooling performance and energy draw.
Doing this necessitates an intake enabling cool air to flow through the fridge’s cooling fins. The consequently heated air must be readily allowed to escape to atmosphere.
The sketches show how – and how not to do this. It is harder to do in a camper trailer but no fridge will work as intended unless it is installed such that it has some source of coolish air, and that heat from it can escape.
That shown here relates to door opening units, the basic concepts apply also to chest opening units.
Off-road use may necessitate using a Donaldson centrifugal air cleaner, possibly assisted by a fan to provide a supply of clean fresh air into the whole trailer’s interior. Arrange an air exit such that the cool air flows as shown.
When installing include an easily accessible on/off switch so the pump can be turned off when leaving the trailer. How to install them correctly is shown below.