Check up on the roof to see if it's been maintained and properly caulked, any noticeable soft spots walking on it. Without removing the interior ceiling, the only other clue you have about water damage is your nose. Sniff for mold and rot smells near the staining.
Drive it to decide it's handling and ride, generally, coming from experience driving lots of school buses, Chevy's ride better than Fords.
(I know I'll get slammed for that comment, but thousands of miles in the driver's seat of Ford, Chevy, GMC and International school buses, that's my opinion and I'm sticking to it.)
Check the tire manufacturer codes, tires 5-6 years old should be replaced within a year, on RVs the tires age out before they wear out.
Listen for leaking sounds around the manifolds and exhaust, cracked headers or rusted exhaust are things to check for.
Have every system demonstrated, generator, A/C, furnace, water heater, etc. make sure they all work as they should.