Unfortunately, rebracing the floor probably won't solve the problem.
I didn't know how they made the floors in these lightweight units, and if I had known, I probably wouldn't have bought one. It is ridiculous to use such a thin plywood over styrofoam and expect it to stand up to normal use. We are very careful with ours, and never wear shoes inside. And we never, EVER, step heavily coming down the stairs from the bedroom.
Over time, in a heavy traffic spot, the thin plywood will press down and compress the styrofoam, and you end up with a "soft spot".
The only real solution I can see is too fill the soft areas with a filler, like that used under vinyl flooring, and then to use a good heavy laminate flooring. The laminate will distribute the weight, basically taking the load of the heavier plywood that should have been there in the first place.
These lightweight trailers appear to be constructed to be disposable, like everything else we buy these days. Use it for ten years, then park it in the bush. Ours will last much longer, but only because we baby it. That shouldn't be necessary.