Just my opinion, but you couldn't pay me to take an old carbureted motorhome.
The dash AC not working, yes it could just need a charge. But it could also be a bad compressor and the entire system is full of metal shavings meaning a complete replacement of all components.
As far as adding an AC, yes they go straight through a standard size 14" vent opening. If there is no AC on the roof then yes you might get lucky and the wires are already there under the trim. But if there is already one AC on the roof I find it highly unlikely that the factory left the wires for a second AC in a 20ft motorhome.
As far as the brand. At that age the model and brand is not important. Your focus should be on inspecting for leaks. Touch and poke and prod and step on every inch of the floor. Any soft spots and walk away.
Look in the overhead cabinets and into all the corners for staining or other signs of leaks. Inspect the roof for signs that the owner has been keeping up on repairing any cracks in the caulking.
If any signs of leaks I would walk away.