In the national parks I've been too, my 30' was fine. When I was buying it back in 2004, I was led to believe that 30' was a maximum.
But, I'd be more concerned with internal design for a trip that length. You don't say how many of you there are, but a lot of the shorter RV's don't have a separate bedroom - just a curtain at the end of the bed. We love having a sliding door to the bedroom as well as being able to block off the hallway for an expanded bathroom (nice when showering - you can step into the hallway where the closet/dresser drawers are and change there).
If you haven't been RV'ing before, rent something now for a weekend and see what you like/don't like about your rental.
JMHO! Good luck with your purchase!
Bill
2004 Bigfoot 29G (Love the garage!)
2017 Northern-Lite 10'2" SE on F-350 (CC, Dually, 4WD, Boss)