Different vehicles, mh and trailer, but from the same time frame and sound similar in construction. My guess about assembly order from tearing mine apart: lay the floor, put the channel on it, ribs and then skin. On the mh I would guess the front was built as a unit and then set in place because there are some heavy extrusions welded into a cage. You can make a guess as to if your end sections were assembled on the frame or a jig. I doubt if the whole body is built and then set in place, to much chance for damage moving and installing.