It looks as if the whole problem has been due to the passenger side window drain hose being blocked. I managed eventually to get thin wire down it and it drains fine, but must have been blocked for years and years and years......
This seems to be something to check on an annual basis at least, and is very easy. The drain hole is visible in the window channel in the front section and 3 foot of thin wire should wiggle its way down there..... I squirted silicon grease down the hole to help the wire on its way.
I removed the vinyl panel
Shouldn't have.... didn't need to. Now I'll have a lot of rivets to replace. Actually I shall attempt to replace the rivets with screws, so that the job will be easy next time. To remove the panel, you have to drill out the vertical line of rivets at the rear edge of the panel, and then remove (gently, with a thin, flat paint scraper) the thin woodgrain plastic, then remove the central plastic panel behind the woodgrain, then drill out all the top horizontal line of rivets. The panel then lifts up for pulling outwards. There are more rivets at the front that I did not bother removing, because the panel will now swing out for access. Don't do it unless you have to.
This was a nice motorhome when I bought it.... but it doesn't look so pretty now!
...... but at least the driver's side doesn't look bad at all.
Almost at the shopvac stage. A little more disaster restoration than I had anticipated.