[rhagfo]

Yes OMG that is ugly!

I had an 86 King of the Road we bought very used with a soft floor. Peeled the walls up past the floor line and slid in a new section of floor.
Surprised that you attacked from the inside, I found it easier to peel up the siding from the outside.
Ended up replacing half of the back wall and lower part of the drivers side wall.
As ugly as it looks if you have the skills with woodworking the cost is not that bad. It is the time that is involved.

Looks like you have/had a bad roof leak, that should have been somewhat easy for the dealer to locate.