Is there a roof-vent fan anywhere in the rig? If not, I would recommend installing one. Much cheaper than adding an air conditioner. Running the fan to pull air out of the trailer will draw cooler air in from the outside. If you're sleeping in one of the pull-out beds, close all windows everywhere else in the trailer, and close the door(s). Then the incoming air will be pulled in right across your bed. This would work in all but the hottest times of day. It would be especially helpful in the evenings and overnights, if the temps are below 70 F at night.
-Speak