IIRC, East of the Mississippi there is only one state (along the Eastern seaboard) where towing RV doubles is legal. Sorry, I do not remember which state that was.
It has been a while since I researched this, so laws may have changed.
West of the Mississippi, towing RV doubles is legal in most (but not all) states. There are some restrictions as to driving licensing, and some states require the first trailer to be a fifth wheel hitch.
Note that just because you see commercial doubles on the jhighway does not mean that you can do it with an RV.