I had roof damage in a small area but water had not penetrated past the top layer of wood veneer. I slit the rubber roof for access, replaced the veneer using contact cement compatible with the foam layer, then sealed the rubber membrane back up with the contact cement and Eternabond tape.
If water had entered the walls my problem would have been multiplied by 100. RV walls and floors are typically the same lightweight sandwich used on the roof. Trying to recreate that sandwich is next to impossible. Replacing it with typical home construction methods and materials will likely weigh more than your springs and axles can stand. From your pictures it looks like you have had water damage in the walls and floor. There are some decent
products to repair rotten wood if it isn't too far gone. If too much wood has to be replaced it can be an overwhelming job.