frank1.....PPL Parts RV Parts and Accessories for Sale - PPL Motor Homes
has pretty good prices on A/C units and will tell you if you need to change thermostats or not. I emailed them with a similar question when I was looking at replacing a 13500 btu unit with a 15000 tu bunit.
The heat pump is just as loud as the A/C unit and is a consideration if you intend to run it at night or while watching TV. We have two heat pumps and run the one in the room were not using, since they're both ducted the length of the coach. In the evening while watching TV in the main portion of the coach, we turn the bedroom heat pump on.
The way most of the heat pumps are wired into the thermostat, is that once they reach a temp where they can no longer heat the coach, the propane furnace kicks on. This might be one of the questions you need to ask and look into. For me, it wouldn't be a big issue that the furnace comes on when the heat pumps can no longer function, but we don't camp in really cold weather.