I can't offer any help on clearing up that huge stain problem but this is something I use twice a season to prevent something like this from happening. I bought this at a marine accessory store when I first bought my hybrid 5 years ago and their suggestion was to spray it on a rag and apply it to the inside canvas just like your applying wax to a car, let it air dry for a few days before you go camping and you'll be good to go. Now living in the PNW I need a dehumidifier most of the time even while not camping.
When I get home from a trip I remove the mattresses so I can expose the canvas and unzip the screen openings from the inside and run that dehumidifier 24/7 for at least a week or until there's almost no water in the pan.
2010 Jayco Hybrid EXP21M
2013 Toureg TDI