If from a private owner ask for maintenance records. Tire age,cracking,bad wear. Outside paint condition. Roof (climb up and inspect) caulking, condition and hardware on roof. Take covers off air conditioner and inspect. Chassis inspection, possible rust issues ( especially bay framing and flooring). Air up and check air bags and braking system, compressor. Check battery condition, water levels, charge, age.
Check genset and operation. Check all lights. Look for delamination. Window caulking. Proper sealing and operation of slideouts. Drive the coach, does it pull one way or the other, shock condition, brakes, any odd noise, engine operation normal, trans shifting, all gauges operational. Suspension and all components.
Fill with water and hook up to city water. Do all water systems work properly without leaks? Electrical system work properly ( converter of any) tv, inside lights, stereo, remotes.
Of course the engine but your getting good info on that.
Look in all above cabinets for ceiling leaks. Look under cabinets for plumbing leaks.