The basics are Batterries, Tires, & Service Records. Inverters , pumps, A/C systems, Heating , Hot water Heater.
Air systems, Brakes, Seals, Air bags, lights inside and outside.
If it has slides, check for ease of movement in and out. Check the ceilings front to back for any water leaks.
Any error codes on the Detroit Engine DDCC.
See if you can acquire History info from Marathon and Prevost.

Flip every switch, operate every lever, run every system.