OK, so I had a welder come yesterday and weld in a number of new 6061 aluminum 1 1/2 x 2 1/2 x 0.125 cross-members, material for which I purchased and cut to fit. The OEM cross-members in this trailer are "paper-thin", ridiculously thin in my novice opinion. The cross-members cantilever across the wheeled sub-frame, holding the shell walls on the overhanging span. They are bolted to the sub-frame. Fractures had occurred where the bolt passed through the cross-member and in the middle of one cross-member where most people traffic would occur. There were also too few cross-members in my opinion, even for a 'lite' trailer. Comparatively, it is now hopefully "built like a tank" without adding that much more weight.
I've decided to retain the underside metal exterior skin and even replace a section that I tore out because it was worn/corroded through. I'll likely just use some galvanized sheet metal I have already left over from my HVAC days (20 gauge I think).
I didn't remove the partition walls, merely tore the floor out from under them and will trim the bottoms when installing the new flooring (polystyrene insulation with Douglas Fir 5/8 ply treated with Copper II wood preservative separately (not commercial "pressure-treated" ply). Going to start on this phase soon.
