Well - The price in y gut seems low. That may not be a bad thing. I basically stole mine because the couple got tired of staring at it and it hadn't been used in like 5 years.
There are some standard things
First is that the layout must be right for you. There are things to consider like how it will be used. Two people regularly is pretty flexible. 4 people regularly is another thing, 4 people occasionally is also different. Sit in there and imagine being stuck inside for rain for 4 days. Are you gonna go crazy or are there different areas to sit - couch vs. dinette for example.
The galley will be a lot tighter than cooking in a house. There is usually limited counter space. OTOH RV cooking tends to be simpler (maybe) - is it workable for you and yours.
Second is water damage. That age I expect some weeping to be inevitable. Mine has a soft spot (recently discovered) near the base of the shower. I don't think it's leaking. I think it is 25 years of water splashing from the shower and getting past the floor tiles. I also have one soft wall near the door/dinnette. So, press your toes into corners near the vanity and shower. push on the walls (usually near the top).
But bottom line for me - any repaired damage to the roof would likely be a deal breaker. The most common places for RVs to get hit are the top corners - backing in and out of shaded areas and hitting overhead stuff. Some may accept that in an older RV. Another thing is how long you expect to own it - If you only plan to have it 5 years minor water damage may not progress much especially if repaired.
The next "big ticket" items for me are the engine and tranny. It definitely should be driveable. No smoke, grinding, stalling etc. etc. To me on an old vehicle if the engine or tranny goes the vehicle is basically headed to the bone yard. A big block 1990's engine replacement is expensive.
RVs tend not to have rust problems even in the rust belt because they usually are stored in the salt season but I always look at the underbody anyway.
The thing is you are buying a "bus" and you are buying a house. To give you an idea of things (my ballpark guess) A/C unit $2,000, Water heater $700, Refrigerator $900-$1500. Furnace $2000. Tires (assuming 6 replacements) $1800 plus install/labor/alignment. RV shop labor rates have been reported around here in the $100-$150 an hour range. About double that of a car shop.
There is something to be said about an old trailer vs. an old RV. With a trailer your are buying a house on wheels. You can always replace the "motorized" side of things with a new truck or tow vehicle...