I would advise you to at least consider renting one for a few days before jumping in with both feet. There are two reasons for this: first, you may find you have second thoughts about this sort of lifestyle, and finding out with a rental will save a lot of money and hassle. Second, and perhaps more important, you can get a much better idea of what sorts of features to look for with a bit of experience. Sometimes it's the little things that make a decent difference. (Is there a place to put a trash can?)
Getting service for a unit purchased from a private seller is not really harder than getting service for one purchased from a dealer, doubly so if you're on the road and not near your dealer. Even better is to learn how to do at least simple maintenance and service yourself; you can save a lot of money over time if you winterize and recaulk and replace broken baggage door hold-open stays yourself rather than paying someone to do it.
Private sellers may or may not be more honest than dealers, depending on the dealer and the seller. A private seller may also be entirely honest but still clueless about problems with their RV, while one would hope a dealer at least has some idea of where to look for problems and troubles. It's up to you, in either case, to make sure you do a thorough examination to satisfy yourself of the condition of the RV you're buying, and to make sure that any problems are taken care of to your satisfaction. If you don't think you're qualified to do that, it can be money well spent to hire an independent inspector to check things out for you. (That can sometimes be a good idea even if you think you're qualified.)