what supports the walls and roof

My manual also claims aluminum framing in the floor, but it lies. The framing under the door was some sort of compressed cardboard the size of a 2x4 lying on its side, and seems to run the length of the sides. The damaged area is only the width of the door, but unfortunately that must support the weight of a 200lb man stepping into the trailer. The leak was from that little 1/2 inch aluminum decorative C-cap strip that is mounted on the bottom of the outside wall ... where some factory goofball did a substandard caulk job.

Now I am wondering if there is actually aluminum framing under the door frame, and that fibreboardstuff was used as a filler for some unknown reason.