Kent, I have gone all over the country this way, 5th wheel witha 14' enclosed. I have tryed many times to check legality but cannot find much. Anyway, my understanding is that in some state (TX, CA?) it is only good if the first trailer is 5th wheel type or gooseneck? and the second is a bumper type hitch. Also not sure if the electric brakes have to be working on the secon trailer.
I have never been stopped with this setup other than I was not allowed on the NY Thruway. Didn't have time to sit there and argue with then so went a different way. I have heard people say GA won't allow it, but have been thru many times including thru Atlanta.
But, for as long as I have done this, it's getting to the point where I wish I had a nice big 5th garage unit. It sure would eliminate a lot of headaches such as finding parke with long enough spaces, having to fill up on the truck side of truck stops, and finding places to stop and park in general. And, a little information, truckers won't like you. They don't like RV's to begin with and they have to have a CDL plus an endorsement to haul doubles. Something to be aware off!