I know you don't want to be too fancy with this: But your air conditioning bill would be a lot different too....There are a variety of indirect swamp coolers out there: Coolerado is far ahead in terms of technology and I believe would work well in your environment. How much air circulation do you need overall? Have a grill or griddle or similar in your unit?

http://www.coolerado.com/how-coolerado-works/

Or work the principle only on the condensing side of your fridge: might mean you have to cut a hold in the side of your vehicle but essentially a small unit like this would make your fridge think it was in a 75-deg room.