If you haven't already, have them plug everything in (and put water in the tank) so you can check if everything is working. Check if the hot water is working, frig is working, fill up the gray and black tank and then drain it. Check for any leaks at the sinks and shower. Run the A/C and Furnace, look at the dates on the tires to make sure they aren't to old and check the pressure. Open and close the slideouts. Check every electric outlet to make sure they work, kind of like doing a home inspection. I'm sure there is more you can do, but that is just off the top of my head.
Dave, U.S.A.F. (Ret)
2019 Fleetwood Discovery LXE 40M (Big D)
2016 Ford Focus, flat tow