I'm sure many will disagree, but, to me, I would never go bigger than 40' unless it was a high end model that has rear wheel steering assist. After one cross country trip with a friend we both agreed that the 45' Allegro Bus we were traveling in was too long. It is so easy to get yourself into a bind in cities. You will basicly have to stay with truck routes. Many public campgrounds can also present problems.
It is, however, nice to have the space and an extra toilet when you have four people on board.
