This discussion came up before and yes there is a company that makes 53' motorhomes. They make them on a semi-truck chassis and they know they're illegal in many states, but they make them and sell them anyway and it sounds like for the most part, law enforcement leaves them alone. At least that is the stories I saw told on various websites about people who have them and claim never to have received a ticket.
And this is one of those violations similar to double towing, where just because your RV is registered in a state where over 45' is legal, doesn't mean you can drive it in states where it is not legal.