If your top priority is staying at federal parks, at least the few I have been to 25 feet would be ideal. We are at 32 feet with our TT and managed to get into a site each time although it was pretty tight, never had any issues at the state parks here in the four corner states.
It is all about your personal taste when it comes to purchasing a new RV, getting into federal parks is not even in our top five priorities. We live and do most of our RVing in the mountain west, we prefer boondocking where roads can be very tight, still our next rig will be longer.
