After reading your one other post about setting it up on property to live in, you should not be concerned with wiring and plumbing other than to have it filled up, check the pumps stay charged and there is no leaks, tanks and dump valves work.
But the most important thing always is the roof and the seams and openings. If a rig is maintained, you should be able to go into it in a pounding rainstorm and feel the walls. They should feel dry. Keep feeling all around every place you can reach with your hands, under every window, roof opening, wall edge.
If you have been reading this forum you will now know which models are highly respected, and which ones tend to have more problems with construction. Weight per foot of length is often a good indication of quality, wall thickness, and strength. Compare weight, then look at top models compared to less expensive models so you know the difference.