If you get a portable A/C, try to find one with a two-hose setup -- a separate air intake and exhaust for the condenser. Single-hose portables take air to cool the condenser from inside the van and exhaust it outside, which means you're constantly pulling in outside air.
I'm putting together a cheap bucket-style portable evaporative water cooler (aka "swamp cooler") to try out on my next trip, but I frequently camp in desert areas with humidity in the low teens or even single digits. I don't have a generator and camp in places without electrical hookups, so this is pretty much the only form of cooling with a low enough power draw to be practical for me; the one I'm building will draw around 20 watts, which is reasonable to support with batteries. However, evaporative coolers have some important constraints; they're one-pass (you don't recirculate the air, you need to keep pulling in fresh from outside), they require quite a bit of water, and they only work when the humidity is very low.
1990 E-250 Sportsmobile w/ penthouse top, converted when new by SMB Texas.