Basically, with out grid power and an AC unit, or running the engine and the AC, the best one can hope for is to keep the interior below ambient temperature until midday or so, at ambient temperature until later afternoon, and not much above ambient temperature as the sun sets and things cool down.<br><br>Keep the sunlight from getting inside. Tight fitting reflective window shades, a light colored Vehicle body, sufficient insulation, and more than enough ventilation exhausting hot air out the roof and an intake fan on the shady side of the van. Combine this with a fan to aim at one's body enough solar wattage to power the fans 24/7, and in Florida or similar, in the summer, you will still be miserably sticky and overheated.<br><br>Low humidity heat can be battled with evaporative cooling. Fighting High humidity heat is nearly futile, best you can do is not make it worse by taking refuge in an unventilated oven.