Pop-up campers, sometimes called tent campers or tent trailers, comprise a lower section which is rigid -- referred to as the "base box" -- and a collapsible upper section. Because of the materials used to make the collapsible structure they are vulnerable to overheating. Cool your camper down by fitting ice coolers operating 12-volt fans or a 120-volt air conditioner running from an inverter, generator or power hook-up.
Buy a small window air conditioner. A unit that produces 5,000 BTUs of cooled air is adequate for almost any size of pop-up camper, and commonly weighs less than 20 kg (45 lbs). Supporting such a unit is easy, but the fabrication must be sturdy enough to resist the added factor of sway when the vehicle is in motion.
Determine the best location for the air conditioner. It must be sited in the rigid base box -- the top wouldn't collapse if you put it somewhere else. It must not obstruct the function of the spring-loaded actuating devices in the corners, which facilitate the cranking up and down of the collapsible top. Locate the unit at the side or back of the camper -- because at the front, water would be forced inside when driving in rain -- and immediately above a frame rail that will support the weight both of the AC and of the fabrication.
Mark the location where the cut is to be made on the exterior wall using a permanent marker pen. Use an angle grinder with a cutting wheel to make the opening. If your pop-up camper has fibreglass insulation, wear protective clothing including a properly rated dust mask, eyewear and a long-sleeved shirt.
Locate the camper's support hoops using a regular household stud finder. Mount a shelf, sized to accommodate the unit using two "L" shaped brackets which are in turn fixed to the support hoops. Locate the outermost lip of the shelf so that it is resting on the bottom of the hole, not butted to the interior of the wall, as this orientation will help with weight bearing.
Reinforce the fitting using straps attached to the innermost part of the shelf and angled up to the highest attachment point available on the rigid base.
Lift the air conditioner into place, and lock it in place using securing mechanisms such as ties, ratchet straps or permanent brackets bridged between the shelf and a part of the AC unit where it is safe to drive screws through the outer case.
Seal around the external rim of the hole using a proprietary self-adhesive weatherproof sealing strip. Seal around the inside of the hole using expanding builder's foam.
Find the air conditioner's drain port for condensed water, and run a hose from it to the exterior of the camper. Ensure vented water will not fall on vulnerable components, or create a puddle where you enter and exit the camper.
Route the power cable from the air conditioner to its outlet in such a way that a tripping or tugging hazard is not created, and secure it in place using plastic cable ties.
If circumstances prevent the use of a freestanding AC unit mounted as suggested, consider placing the unit inside the vehicle. Use ducting made from flexible hose sold to vent tumble dryers, and a bathroom exhaust fan positioned to remove the heated and humidified exhaust air. Panel around the ducting to match the interior of your pop-up camper, then insulate the tunnel to prevent inadvertent heat transfer back into the camper. Have a drain to evacuate the condensed water.