Check the laws in every state that you will be traveling thru. Double towing is prohibited in many states. Backing up is like pushing a rope. Also, read the fine print on your 5th wheel warranty. Any modification to the frame may void any coverage.
Just a thought but there are at least two companies that make special kayak and bicycle racks that mount on the rear of a trailer. A fishing kayak with a trolling motor is not a bad substitute for a boat with an outboard.