I'm not so sure about the 10 year rule. It REALLY depends upon how the previous owner took care of the unit. A camper that has sit for 10 years doing nothing but being closed up may be in worse shape that one that someone full timed in for 10 years, the exact same model.
There's a real balancing act with any RV between age, upkeep maintenance, and expense. But no matter what RV you purchase, or no matter how new (or old) it is, if you are going to maintain them, it will cost you something! I think the only time an RV does not cost you anything is when they are still sitting on the dealer's lot! The minute you sign the dotted line .... you start parting with your $$$. And if you really want to keep your camper in tip-top-shape be prepared to part with some of the $$$.
People trade or sell RVs for a number of reasons.
1. They get too old and they just want something newer, or they get board with their current model. They look for something exciting again, and buying a new RV IS exciting!
2. The repairs start to accumulate and the costs begin to climb. Rather than investing another penny into a failing camper, they sell while they still have some sanity left!
3. Needs change. You start with a pop-up and soon realize your tired of wet canvas. You trade and get your first TT only to realize shortly it's really too small, so you trade again and get something bigger, only to find out the floor plan really sucks, so you trade again, and again and again.
4. There's a natural progression as your income level increases with your age: PUP, Hybrid, TT, 5er, Toy Hauler, CLASS-C, MH.
5. Family sizes change and you outgrow your current camper, then when the kids leave, you down-size again.
6. You simply get too old and you cannot continue the RV life style (either week-end warrior or full timer), it just becomes too much for your age.
7. Some people end up with serious health issues that drive them out of the RV business, so they sell their campers.
For all the reasons above, a 10 year old camper may be very good, except for the last category:
8. Some people just really don't care. They abuse their campers and they quickly become junk. Everything falls apart, and then they complain because they cannot get top dollar when they try to dump it on some one else. These are the one's you have to watch out for, and usually there are tall signs of abuse or neglect. Rotten cabinets, rotten counter tops, soft and rotten floors, water spots on the roof, indicating leaks (run if you see this). More than likely, if you see interior damage in the construction, the furniture, floor, windows, holes in the walls, rotten water lines, electric lines, propane gas lines, anything like this ... will be a GOOD indication the appliances are bad also..... Run!
Before buying used, try to find out why the previous owner sold! This helps a lot.