I have not had any issues with law enforcement. However, I have stayed out of the states that specifically state they do not allow double trailers. These would be WA, OR, and all states on the Atlantic Seaboard. Federal law does state you can pull on the interstates, and have to be allowed on local roads for fuel, food, and lodging. I just haven't pushed it as I haven't had any need to pull the 5th wheel to those states yet.
I purchased a separate plate for my trailer. Got a permanent plate, same as they put on 18 wheeler trailers. Cost $90, but never has to be renewed. I have seen one instance where the 5th wheel plate was put on the trailer. I believe the guy was from Ohio, but not really sure.
I had no sticker regarding warranty on the frame, but would have installed the hitch anyway. Not installing would mean not taking the bike with me. I had the hitch fabricated at a local welding shop, and they did an outstanding job. If anything, the frame is now a lot stronger than it was. Cost a small fortune, but it was done right.
If you run across me in AZ this winter, be sure and stop by for a cold one and a ride planning session.