To answer your original question, I would NOT use that rear bumper on any
5'ther for anything other then a short 2 place bike rack if that! That back bumper is simply ment for storage for the dump tube and will not handle the braking of 3,000 lbs from your boat for sure, I have seen 2 boats that tried to enter the back of the 5ther because the bumber tore--the metal on mine I bet is not more then.040+/- thick. I modified a class 3 Reese style hitch off a 4x4 ranger truck and it fit the back of my trailer perfectly. Bolted to the bottom of the frame rails of the trailer, used the bottom of the dump tube "bumper" to triangleate the setup and it was all set to go. I think I had a wopping $50.00 into the whole setup.
In Michigan there is a requirement for a special endorcement to your license that you can take at any sec. of state.
Each State seems to be drastically different in the offerings so check into each state you will be driving into.
In Michigan your first trailer MUST be a 5th wheel NOT a goose neck (interesting) and the last trailer can be the convential ball type.
I had to laugh, over the weekend, I seen a RV pulling a Jeep Cherokee using a simple ball pull, pulling a bass boat being introduced to a State Police Officer----Obviously that rig was not legal on Michigan roads.
I do agree that my commercial drivers license for doubles allows me to pulls doubles on a daily basis.