My trailer floor is a sandwich of 1/4" plywood with 1 1/2" Styrofoam in between. The lower plywood got wet and separated from the Styrofoam it was glued to. This left the floor springy where it once felt solid. Glued under the bottom plywood there is/was a black Tyvek covering glued to the lower which is what one sees if you look under the trailer. I could see and feel the Tyvek hanging down in the soft area when compared the the rest of the trailer. Your's may or may not be the same but it's one way of checking. And the floors CAN be repaired, it's pricey and you have to find a shop willing to do it.
If the floor is shot an good option is calling it in to your insurer, I wish I had.
I would have to drop the fresh water tank either weld or bolt in a cross member, which it needs. Don't think I'd trust JB Weld to hold it in place. If it broke loose on the highway it could be a big problem.
Happy Motoring
Bob & Deb
W Ma.
12 F150 HD SCAB EcoBoost LB 4x4
14 Escape 5.0 TA