Bug Badly Bumped bumper needs replaced.

I'm afraid I am govong up on the bumper for now. Was about to start pulling the windows to reseal to keep water out and and tint them while I was at it. thought I'd pull the hatches too (small hatches, no basement) and found tat the sides are badly rotted. Apparently the sidewalls are made with aluminum and foam sheeting. These are covered with a thin sheet of ply inside and out. The bottom edge trim piece is made so that it collects water add keeps the ply wet. The wet wicks up. The bottom foot or more of the outside ply is rotted to nothing. On the inside it is even better. The inside sheet of thin ply is covered with a thin black fabric matting that holds moisture against the ply layer. It is shot nearly up to the floor. I'm pulling the hatches and the storage buns to figure the full extent of the rot. It reminds me of a boat that has been submerged for years. Why do they build these things this way? I would have been cheaper to make them so they would not rot actually!I'll get back to the bumper when I figure out how to fix this. The good news is that the outer skin is fine and still attached farther up the sides. The bad news is (see the good news) I'll search DIY for clues.