Condition is everything when it comes to an RV of any type or age, here are a few issues you need to be aware of:
1, Rot is the primary killer of motorhomes, this usually occurs due to water intrusion from the roof, sealant around the roof penetrations (vents, end caps, etc.), as well as around window frames (commonly around the above cab bunk windows on class C's). Rot is often far more extensive than it appears at first sign, and far more expensive to repair than most people think.
2, Cosmetics may matter on some older coaches, more and more commercial RV parks are implementing a 10 year old rule, meaning coaches over 10 years old are not allowed, or must submit photos proving they are in good condition. Enforcement of these rules are often spotty, as the real purpose is to keep junky looking RV's that may break down and be stranded at any moment out of the rv parks. Many parks also only apply these rules to longer term residents staying a week or more, personally I have not ran into the issue, but others have.
3, There are a lot of rubber components on an RV that will age out somewhere in the 10-15 year age range, this includes not only tires (which most people suggest should be changed every 7 years regardless of wear level), but also belts, hoses, and numerous suspension and steering bushings. Here it helps to buy a coach that has been well maintained, and where the previous owner has already done some / most of this potentially differed maintenance.
4, Inspect, Inspect, Inspect, buying a used RV is not like buying a used car, a professional detailed RV inspection on a 30 ft class C should take on the order of 5-6 hours to complete, and cost somewhere around $600-700, depending on the specific details, location, etc. this does not include a detailed inspection of the engine and power train which should be done by a mechanic familiar with this specific brand of chassis. Buying an RV is much more like buying a house, except in this case it is the house, with all of the appliances, that have spent their life in harsh conditions, often without climate control running, and being subject to a rolling earthquake every time they are moved.