As anticipated. You can hardly expect a manufacturer of a trailer designed, built, and tested for a bed mount hitch to encourage anything else. Product liability is one reason.
Bumper pull trailers are balanced to carry 10-12% of their
weight on the hitch. Fifth wheel and other over-the-axle trailers are balanced to carry 15-20% hitch
weight. They are not interchangeable.
Afraid you're on your own on this one.