It's been a lot of years since I had a popup, but one thing they have going for them is that there are a lot less things to go wrong.
Other than tires and suspension, I imagine one of the more important things to check would be the lifting mechanism. How smoothly does it operate ? Any noticeable issues/problems with the cables, gears or whatnot?
What condition is the canvas,vinyl,sunbrella, evolution3 or whatever fabric used on the unit ? How are the windows and screens ? If it has an air conditioner, does it work? Any signs of leaks in the roof or sidewalls?
Does it open and close like it should?