Victory! Ceiling stain is 99.9% gone.
As wonderful as I think a solid roof is... the end caps you'd think would be designed to mate with an overlap to prevent leaks. The ones on my 2002 Sunseeker seem to more rely on an owner paying attention. All four corners needed assistance. I'm pretty sure the reason this was unsold for about 4 months was because of the reaction of people (like my wife) Eeeewue.. "This leaks." Ya..?! (if you've ever been around a boat...)
In a prior posting, I was on another crusade to turn yellow old plastic into new. That worked somewhat well; this worked much better.
Same routine: 40% Hydrogen Peroxide cream from Sally Beauty Supply .. a 20W UV lamp from Amazon .. and time. I did not clock this but would guess about 30hrs of lamp time. I was unable to keep it wet with Saran Wrap, so I spritzed it with water and applied more with a toothbrush.
I should try it on the carpet, but it is overall pretty nasty so we are looking into boat decking and/or some new woven vinyl flooring both from the marine industry ...to get rid of the shag and Linoleum combo.