Check the tire date codes, make sure that they are recently replaced, and not original tires.
Also, check all chassis hoses that you can visually inspect. Your coach is 12+ years old, (probably built in 2002), and the rubber/plastic parts have all deteriorated to some extent. Also check all the window seals.
Test the water system. Use the water pump, and also connect to city water. Look for leaks under the cabinets and at the water center outside.
Check the electrical systems. Run the generator under full load, (all A/C units and microwave, and lights on at the same time). Make sure the batteries are in good shape. Make sure the batteries are charging when connected to shore power.
And, carefully examine the exterior body for leaks, repairs, and assembly. The entire rear endcap of my shiny brand new Winnebago motorhome fell off while still covered under warranty. A used 12+ year old motorhome has experienced alot of rattling in its life.
Most dealerships selling a used motorhome, (especially a Winnie), make sure the floors, carpeting, cabinets, and bathroom, and vinyl are all clean. But check drawer slides, hinges, light switches, locks, pocket doors, latches. Make sure all "fold out" items are intact with no missing parts such as necessary legs, mattresses, tables, etc...
Make sure all cabinet gaps and warping due to use and age, (to each other and the ceiling), have been repaired. Make sure all floor, walls, ceiling, and woodwork nicks, cracks, scratches, and gouges have been repaired.