"I recently bought a van and also I'm struggling with the heat.
How did you solve your problem? Or maybe anyone has some other bits of advice?"
alot of foam insulation and swampcooler is how I handle the heat.
I always stay in the same area and park in the sun (due to solar panel and for convenience) . By insulation I'm talking multiple layers all over the back (living area) of your van. Cover all you windows, build a divider between the front and back of the van. When its noon and your in the back and turn off your lights it should be pitch black.
Once its well insulated, the swampcooler will keep you cool except on the hottest humid days, and even then its better then just a regular fan.
The mini AC's like the zerobreeze use too much power (about 180 watts) , even my 365 watt panel would struggle to keep it running and still top off my batteries. Swampcooler uses about 3 amps which is no big deal for most solar systems.
Without the insulation/swampcooler I couldnt spend more then a few minutes in my van when the sun was out. I only paid 800 for my van, I'm not worry about resale value, thats why I modify the back of my van for my convenience and comfort.
Its summer again and temperature is climbing, but so far I been very cool, I only been running one fan on my swampcooler to stay cool( maybe 1.5 amps). So far been using less then a gallon of water over a 2 day weekend.
This is a picture of the temperature of the front 148f(uninsulated) and back 94f(insulated) of my van on a hot day. Without heavy insulation in the back, they would be about the same temperature. Swampcooler wouldnt be much help in that situation. 94 degrees is high but its cooler where the swampcooler is blowing.