The deal breaker items, as others have mentioned, is water damage in the roof, walls and floors. On the exterior, use a ladder to examine the roof including on top of slideouts. Look for cracked or missing sealant and worn roofing material.
On the interior, look for stains on the ceiling, inside of cabinets, on walls, under windows, and soft spots on floors. Water will sometimes take a not too obvious path from a leaky roof or window down a wall and into floors.
I replaced a roof once by completely ripping up and replacing the roof deck, most of the framework and styrofoam insulation. The rot was much more extensive than was apparent and included in a wall as well. It was my first RV that was older when I bought it and I didn't have a clue. Luckily I had the time and an indoor place to work on it. I would never do it again but I did learn a lot on how these things are put together.