If it was sitting for three years, needs what the above mentioned, and a few more (I don't intend to repeat, but add a few):
- Oil, brake fluid, and ATF changed. Don't forget the generator oil as well.
- Inspection for rodent intrusion in the engine compartment and elsewhere. Too much rodent infiltration may render the rig uninhabitable.
- Generator dropped, new carb replaced and voltage/frequency tested.
- Fridge flue checked out and burner cleaned. It may be a nice home for spiders or even squirrels that might nest there if the opening is large enough.
- Water system sanitized thoroughly. I know in Texas that fresh water in a tank will grow entire civilizations due to the algae unless kept cleaned out.
- Like stated above, check for water intrusion on the inside. If it has soft spots, bubbles in the wallpaper, discolorations, and mold smell, it is time to unload it, as it may not be habitable.
- As above, water heater... If a Suburban, replace the anode rod. If an Atwood, drain and flush. If not winterized, the water heater might be cracked/split.
- The coach battery and the house batteries are almost certainly shot. If not turned off or disconnected, parasitic discharge has probably killed them long ago.
- The propane system likely needs testing, from the tank to the appliances.
- Tires have a good chance to be shot, unless they were replaced before the motorhome sat.
- Check bedding and sleeping surfaces. They may be too moldy to sleep on. Same with the cushions on the sofa and dinette.