Naturally the first order of business is to stop all leaks so that probably means starting from the outside. Next if you are going to have to rebuild any wood framing you need to work from the outside removing siding, roofing and then the framing while you measure and copy each section replace and recover. Both from experience and from what I have read from others, working from the inside just does not work.
Brian, Loretta & Daisy (Golden Retriever)
2008 Holiday Rambler Endeavor 40 PDQ , ISL 400
2014 Ford Explorer toad