I believe Utah has a max length of 65 ft.
Many have said no problem if over 65ft as the law is not always enforced.
But as has been mentioned if you were in an accident regardless of who was at fault and you were over length then all bets are off in this law sue society.
I highly recommend brakes on the boat trailer which it most likely already has surge brakes. If not recommend brakes as it will not take much for an 1800 lb boat and trailer to turn the 5th wheel sideways.
I tow a utility trailer behind my 5th wheel with no problem.
But it is easy to forget the trailer is there as it can not be seen in your mirrors. I use a camera on the back of the 5th wheel to check on the trailer and make sure it is not swaying etc.
I have TPMS sensors on the utility trailer tires as a flat on the utility trailer will never be known until someone flags you down.