Before you buy ANY used RV--motorhome or trailer--you need to have it inspected by someone who knows what to look for. You can find such a person by asking around at nearby RV parks for RV mobile repairmen or on this forum, but you will have to give your location. This rule also applies to RVs sold at dealers.
With a 20 year old RV, you can end up with something that will cost you thousands of dollars to fix--sometimes more than the RV is worth and more than you agreed to pay for it. The inspection may cost you a couple of hundred dollars, but that is a LOT less expensive than being ripped off or finding thousands of dollars of hidden repairs needed before it is usable.