Start by taking the vents and other stuff off, then start at which ever end has the extrusion, probably the rear, then the side rails and all those screws. Hopefully you have a dry shelter to park it under! You will probably end up having to replace most of the roof frames as well as some of the wall top plate. Good thing lumber is cheap.
FWIW I have toyed with the idea of doing RV repair as a retirement job, but no one really wants to pay to have things done properly.
I have two Airstreams that I am rehabbing right now, the main one is taking longer than expected so we bought the Coleman CM16FBS as something to use until the AS is done.