Make sure the van is insulated like crazy, especially the roof. Park in the shade if possible, keep reflective window coverings over the windshield, with the reflective material touching the window or as close to it as possible. If it's brutal hot, and there's no real chance of rain, place the reflective material on the outside of the window, to prevent the heat from ever getting through the glass in the first place. Have the side windows tinted as darkly as your state allows, and use reflective material on those, as well. Airflow is critical, with either a couple of roof vents or a vent and a cracked side window (will need a screen to keep out the flies and wasps and bees, especially the wasps and bees, I don't like wasps and bees). A Fan-Tastic Fan Vent with a thermostat and a moisture sensor (if it begins to rain while you're sleeping, the vent will automatically close, and will automatically open when it's no longer raining) is a good one to get. Not cheap, tho, but worth every penny. You'll want to hook the fan vent to an aux battery, as even a 1 or 2 amp draw will kill your starter battery if left running for 18, 24 hours.
Evaporative coolers will not work in humid environments, so unless you spend a lot of time in Arizona, don't bother.
Next up is a rooftop air conditioner. You'll need either an RV-camping type with a generator to power it, or you'll need a bunch of deep cycle batteries and a 12-volt DC air conditioner.