Whatever you do, DON’T USE DRY ICE! Dry ice is made from carbon dioxide and as it evaporates in an enclosed space (your van), it can displace the oxygen. You could die!
I’m mostly in dry climates, so a direct type swamp cooler works well. Made it myself from a 20mm surplus ammo can, small water pump with some hose and fittings, a trucker type fan with a rheostat (to vary the fan speed), and a thick MasterCool type pad cut to fit the ammo can. I made up a long cable with the controls at the end of it so I can control it while in bed or at the other end of the van. Air is drawn from beneath the van through a floor register and flexible dryer hose into one end of the swamp cooler, gets pushed through the wet pad (water pump pumps water from the bottom of the ammo can over the top of the pad, keeping the pad wet), and comes out the other side 15 to 20 degrees cooler.