water damage in my caavan?

front seals need replacing, i can do this, but both front side walls feel a bit spongy, iv been looking on web sites but cant find what i need can anyone help. i need new internal walls front and someone told me you can get this lined on boards instead of doing seperate, also what i need to do to make sure water does not come in again, its traveling along awning . a guy just came out said its going to cost me 500 pound