Try to find something with high initial quality. Monanco, Newmar, Tiffin. If you go older, Bluebird, Beaver, Safari. Things seem to hold together longer on these rigs.
Spending a large amount of time in the bus on he proposed trips, floor plan is going to be important. If you decide you need slides, make sure you understand the different manufacturers and which slided they use.
Also keep in mind that prices on RVT and RVTrader tend to be higher than final purchase price. Set aside 10% of the purchase price for things that will need to fixed in the first year.
Do not buy a new or used class A and start on a trip to Alaska the next day. Do some driveway camping and weekend or week long trips to campgrounds close to home to work out what you'll need in the bus to live.