How long is too long? (Over 40 feet - legal?)

Here's an interesting article that further concerns me. This article says that 13 states limit the length of 5th wheels to 40 feet, but is that just for purposes of registration in their state or is that a legal limit for entering their state with your coach?

http://www.woodallscm.com/2012/12/big-fifth-wheels-have-cloudy-future-in-california/