I am one of the lucky owners of a Marquis with basement air. While the primary, basement A/C was replaced by the prior owner, even with a new unit it would only cool at an acceptable level when the outside temperatures were below 85 degrees. In the desert when the temperatures would climb over 100, the inside would hit 90 degrees. Hardly sleeping temperatures.
I say all this to say to resolve this I took the advice of others with basement air and decided to supplement the basement air by removing one of my three fantastic fans and installed a rooftop A/C. The actual installation wasn't too bad except as Bill refers to it, the maneuvering the unit onto the roof. Even with two of us on the ladder the unit was a battle and awkward at best. When I did mine, I did a combination of direct vent registers and piping it into the existing duct work. With the newer 15,000 BTU higher efficiency Dometic Penquin A/C unit, I could get away with running my basement air less and when they are both running, hold 75 degrees regardless of outside temps and sun loads.
If you feel you are up to tackling getting it on the roof the removal and installation is just time consuming. You will want one that will integrated with your existing thermostat but the rest is just a process of unbolting/re-bolting, rewiring, cleaning and sealing. I also would consider doing both, or all three if you are so equipped, at the same time. As these motorhomes age it is only a matter of time till they all need to be replaced.