I replaced mine a couple of years ago with a Carrier 15,000 Btu heat pump, see http://www.airforums.com/forums/f427...ead-40795.html
. Simple—off with one and on with the other. The worst part was scraping the 25 year old sealant off the top.
My Excella used an air conditioner with all the controls built into the system so wiring was just connecting the 120 VAC. You probably have thermostat wiring to contend with, but no matter what you buy you will have to do that.
I am pleased with the upgrade and think you will be too in Biloxi. In full sun it still gets warm inside, but mine does not have a white roof. Another thing on the list.
Since Carrier is exiting the RV business, you will probably want to go to a different brand. Too bad, mine is fine.