Box Falling Apart... How Assembled?

[Kamaloha]

RV: 1989 Leisure Odyssey Fargo Americana.

All, Can I pick your brain? I am trying to decide how to fix what is collapsing in the structure of the RV box. It would appear that some plywood floor on the left side beneath the shower, sink, stove is not original. Have any of you you replaced that? What was the sequence required to do it? What did you discover about how it was put together? I can't figure out what actually supports the sides and roof of the box, ie how it is attached to the floor. Right now what I am seeing is plywood that appears to be in excellent condition under the stove, but it does not go all the way forward and sit on the lateral steel frame member behind the wheels; it is cut about five inches behind the frame member, and it has displaced downward about three inches behind that cut, causing the entire side of the box to also be displaced downward.

On the right side under the fridge, as well as under the bathroom cabinets across the back, the floor plywood is delaminating and unable to bear weight, obviously from long term water intrusion. I will need to figure out how to support the box so I can replace that plywood.

Any tricks you figured out would be appreciated. If it is indeed the damaged floor that supports the box, as it would appear, it's going to be pretty tricky to figure out how to replace that plywood.