Inflatable yellow stain remover ?

[MMcCawley]

Hi, I have an older Quicksilver PVC inflatable that has yellow stains over much of the bottom surface. I've read through the posts where people recommended cleaners (some of which I've tried including Bar Keepers, Soft Scrub, Comet, etc) but I still haven't been able to restore the pure white color (which may be impossible). Does anyone have any magic stain removers that they can recommend?