If you are mechanical and you are buying something simpler, TT vs class A diesel, I think you could do the inspection. But it should take some time - hours not minutes.
Run everything on gas, run everything on 12v, run everything on 120v.
Run water on the tanks & city. Look for leaks - inside, basement, under TT. Dump the gray & black tanks - should only be the water you ran - to be sure the valves open & close ok.
On things like the slides, run them several times. Listen for any odd noises, look for smooth movement.
I would pull the bearing covers and inspect.
The one thing I would for sure pay someone to do is a leak test (https://rvleaks.com
). Roof & window leaks are very common, and also not commonly repaired completely.
However, before you do all that first decide is this the one, based on your initial review/inspection. Agree on price and state, in writing, that deal will close upon completion of the detailed inspection. (A list of everything you will do should be included. ) Any issues over $x (relative to the purchase price) would be negotiated.
One final thing: you really want the owner to do many of these things while you are watching. This way if it breaks or doesn't work, there isn't an issue of who broke it. Plus a great PDI benefit if you buy it.