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)and much broke on mine I just did the calcs and ended up 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 then filled the large holes with sheet foam, the put a rubber pad on top. The mistake I made was in trying to repair the rail beam first, I should have let it sit on my steel frame off the truck for a month or two to get resettled, then repair the beams. With only a two month trip this winter to Mexico it did real well.
If rebuilding I would use a TJI/BCI floor joist which comes in 7-1/2", 9.5", 11-7/8" and 14" depths. Properties are strong, straight, and not much more expensive than structural lumber. Almost any real lumber yard has them, not HD or Lowes.
I'd be happy to help.
