When I sold our old pop up to a buddy, he put a window unit in it. He found a cabinet that was just a little bigger than the unit, put it in there, and then cut an access in the outside of the camper and installed a door with a grille in it. He had 2 problems with this set up though. It was sitting on the floor, and cold air doesn't rise, so it didn't get as cool as they wanted in bed. The hot air from the condenser didn't blow out as well as it should. He tried opening the door on the access he cut and sliding the unit out some to exhaust the heat, and this seemed to help.
If you have the room to carry a stand of some sort, a simple way would be to put the a/c on a stand and just let it blow throught the screen from outside. Might need to hang a small tarp to help with rain???? At least this would let you experiment without cutting up your pop up.