If you really want to double tow, let me give you one more option. Instead of going with a very heavy duty pickup and 5th wheel you can go with a motor home which tows a normal car which is towing the jon boat. That bypasses the problem of the travel trailer not having a good rear hitch and an also winds up with a lower total cost. You just have to make sure the toad (vehicle being towed) is capable of being towed flat (most auto-transmission 2wd vehicles CANNOT be towed flat).
Let's step this game up a notch. Get a over/under trailer for the car and boat. Then pull one trailer behind the RV. Legal in all states.
HOI Crappie Club
Where family and friends come to compete for a little more than bragging rights.
Quick, someone teach me how to fish so I can win this tournament!!!