One thing that hasn't been mentioned here is that depending upon state moving up beyond 40 foot may require a different license. In CA for sure anything over 40 foot requires a class B non-commercial license, which requires doctor physical every two years.
Although I know a lot of owners who drive without the proper license, I would personally hate to be on the receiving end of the lawyers questions as to why I was driving a 40' something foot vehicle and wasn't licensed to do so. Could be nasty.
Note, that other states have different requirements. Some states require a different license if over 26,000 pounds, or has air brakes etc. Please check your local state regulations and make sure you are legal. The RV dealers will generally not mention this detail to you.
Ray