Being a mechanical engineer with a little structural experience, when one of the main rail beams(two 2x2's with some 1/8"/3mm plywood on each side) broke on mine I just did the calcs and ended up building a subframe with two 1-1/2" sq 16 ga steel tubes(side by side not top and bottom) under each side all welded up. I used some cross pieces and had to build around the drain pipe. When complete and painted with POR-15, I filled the large holes with sheet foam, then put a rubber pad on top. It is light enough for two small people to easily pick up to put in the truck and strong enough to support a very heavy camper with a long overhang.
If rebuilding I would use a TJI/BCI floor/roof joist which comes in 7-1/2", 9.5", 11-7/8" and 14" depths. Properties are strong, straight,won't warp and not much more expensive than structural lumber. Almost any real lumber yard has them, but not HD or Lowes.
I'd be happy to help.
