after seeing the pics, it reminds me alot of what a friend started with on his trailer. but his started out a bumper pull camper trailer.
he built a "C" channel frame on top of the original frame out of 4" C channel. the run 4 the length of it, 2 on the outsides and 2 spaced 3' (maybe) apart over the middle. then he run cross braces every 2 foot, with every other one running the full width. so it would start with a 7'6" peice then a 2' in between each of the two outside rails then another 7'6" and so on to the back. he stiched welded it to the main frame every 6-8" or so, even to the cheesy/weeny edge support things.
he then took 1/8" diamond plate (i think it was 1/8") and run it between the two outside rails the full length leaving the center open. he also made drive over fender for it, but he made it 45* sides and found it to be too hard to drive over it (front tires hit the fenders as the rear tires hit the ramps (had to bump it most of the time which was sometimes scary, that was untill he got a winch for it)
it was a sweet trailer, but the problem he had was he only built it 18' with a 1' beaver tail for a longbed truck which is 18' long. he didn't really plan that part out. it was very strong. not really sure how you would tie it into the 5th wheel hitch with out looking at it in person.