It's legal in about 21 states. Of those some have other restrictions, such as two communications with the driver, safety glass, etc. It's not something I would do because, unlike a motorhome, there are no seat belts in a fifth wheel. I'm sure it also gives a rather rough ride because of the location of the axles.
Bob and Joyce
2013 CC Silverback 29RL
2010 Ford F250 XL Crew Cab 6.4 liter diesel
Honda 2000i genny