I have enertained the idea of building one of these! A cooler full of ice water, small water pump, a condenser or radiator and a 12v fan. I made a small one and it works surprisingly well. My only concern is how long the ice will last. Unless the cooler is very well insulated not long I imagine. I am going to do some tests this week and see how long I can make a 10lb bag last. I will post up some pics if I get it to work well.
I am actually working out a design in my head that will incorporate dry ice, lots of metal tubing and antifreeze. My thought is that getting the antifreeze close to it's freezing point will allow me to blow low velocity air across cooling fins leaving much of the cold in the returning antifreeze thereby helping to make the dry ice last longer. The main hurdle will be insulating the box well enough to greatly reduce evap. time because either the entire unit needs to be kept outside or the off-gas exhaust will need to be plumbed to the outside of the camper.
Of course, i haven't searched for or priced dry ice in my area so it may be cost prohibitive.