I know I'm a little late jumping in on this one, but:
I've seen the window unit ACs hanging on the outside of RVs. If fact, one almost hit my car when a neighbor's unit fell off. The nature of RVs prevents a window unit from being properly secured. The neighbor's unit was braced from underneath (to the ground). When the RV rocks, the bracing loosens... and eventually fails. So, besides looking tacky, they're dangerous when not properly mounted. I'd recommend an RV AC unit mounted in the location for which it was designed. It may take time, but the wires CAN be snaked through the ceiling and down a wall to the breaker box (for 50A units) or arranged somewhere so that a heavy duty extension cord can be run from it to the power post (for 30A units).
Just my 2 cents.