>>1003732A/C is going to use a massive amount of electricity. You could probably rig up an evaporative cooler (NOT a swamp cooler) to the roof or back window.
Basically, it's just an aluminium heatsink or panel that replaces the window, with either capillary grooves cut into it or a towel glued to it. Water flows from a bottle, through the panel, into a small trough, then up the grooves or towel through capillary action, where it evaporates. The evaporative action removes energy from the water in the form of heat, cooling the aluminium and in turn, the air on the other side that comes into contact with it.
Depending on the temperature, wind and relative humidity, as long as the aluminium plate is shaded, it should provide more than enough cooling for such a small space, and cost nothing for upkeep.
You might even be able to rig up a PC fan powered by a seebeck module utilizing the temperature differential between the aluminium plate and the ambient air, albeit at a small sacrifice to the cooling efficacy, in order to help exchange the air in the van across the heatsink, which would cool the van down even faster.
