I think the original question is very personal. Some buy new, others buy used, some trade in, others sell privately..........
Your original objectives are to maximize trade-in AND avoid problems. As others said, most RV today are pretty good IF they are maintained well and not abused, they will easily last 10 years. My TT has been stored under cover and gets washed inside/outside after every trip - it still looks like new.
Having said that tough, a RV dealer also has to make a living, so you either get a good deal on the purchase or on the trade-in but rarely both!!!
You've paid for the bigger part of the depreciation in the first 3 years - so if the RV is in good condition - keep it and enjoy it.
Gerry
Gerry