I agree with what Bordercollie said.
We also bought an old RV. It was 19 years old when we bought it, it is now 24 years old. It is a 1990 Winnebago Warrior.
The effective lifespan of RVs is about 15 years. After that, they start falling apart. If they are garage kept this can be extended.
Older RVs are going to probably have rubber roofs instead of the nice fiberglass roofs of today.
Trust your nose. If you walk in and smell mold or rot, walk away. Water damage is often what takes down RVs and repairing it can quickly cost more than the RV is worth. Water damage is almost always more extensive than what you initially see from the outside, and ripping out the damage quickly becomes a money pit.
Another problem with older RVs is the lack of replacement parts. My 1990 E350 cruise control motor cannot be found anymore, so I don't have cruise control anymore.
If you are, like we were, in a tight spot when buying, be aware that you will probably be shelling out a few thousand dollars right off the bat to fix things.
We bought our RV for $7500, but had to put $1000 into new tires, $500 to fix the generator, $1000 into the cab AC, which had to be converted to the new refrigerant, and more.
Steve