I installed a second AC unit in our previous coach, a 29 foot Thor ACE. As Clyon51 said, the ceiling vent hole is a standard size BUT you should check with the manufacturer for roof loading. A typical 13.5 AC unit weighs in excess of 100 pounds. If the coach is not prewired, you'll need to run a 20 amp 120 volt line, and most units need 12 volts as well. I used surface mounted plastic wiring channel, not very pretty but a whole lot easier than pulling cable through the walls. The simplest installation would be to install a 20 amp breaker box and run a drop cord from the 20 amp outlet on the campsite pedestal to it.
Bruce Dickson 2013 Thor Challenger 37GT, 5 Star Tune, Safe-T-Plus Steering Control with Air Trim, Roadmaster front and rear Sway Bars, SuperSteer rear Track Bar, Crossfires, 2012 Honda CRV on Blue Ox Aventa LX tow bar. Full timers since Jan 2012.