1. HVAC [Heating, Venting, and Air Conditioning] (Dash, both rooftop units, auxiliary rear, and both furnaces.
3. Toilet functionality.
4. Verify air admittance valves (AAV) are good (if not you will get a sewer smell when the coach is sealed up and you turn on the ceiling vent fan to exhaust)
5. Ceiling vent fans (all of them to include the bathroom)
6. May try to have them change all the internal 12v
bulbs with LEDs. It is something you would do anyway over time, and so might as well have them cover it.
7. Ensure the gauges on the dash work.
8. Verify the black water tank flush if equipped is working properly and not clogged.
9. Ensure the generator has a full service completed to include, filters, carb rebuild, oil change.
10. The brakes should be a considerable concern, either check yourself, or leave to the mechanic.
11. When you test drive remember the P30 chassis is a bit loose on the steering side, but you could play it to your advantage requiring the super steer bell cranks to tighten it up.
12. Go over the electric carefully.
13. Check the water pump for age, leaks, and consider having them replace it anyway, and consider requiring them to keep the old one for a pinch.
14. Also have the coach appraised by someone with no stake in the deal. The appraisal can be used for cash value insurance for the first two years after the purchase in most states regardless of age. Also can be used to further negotiate.
Let me know if there is anything else I can do to help.