Are you hoping that the fridge vent will provide enough opening to exhaust the heat generated by the AC? I am not sure. Hot air rises, and besides, the fridge needs that exhaust opening or it may not cool as well as it would otherwise. And remember that condensation will form back there from the AC... where is that dripping water going to go? Just some things to think about.
I had a 16' x 7' KZ trailer for a while, and it came with a 5000 btu wall unit mounted up high with a dedicated grille opening on the outside wall. The trailer needed to be parked with at least a slight slope to that side so the water would run out and down the trailer's exterior, otherwise it could run inside and make a mess. Also the unit was woefully inadequate in weather over 90 degrees or so; 8000 btu would have been better.