Dang, son, I'd expect someone from Wildwood to come up with an answer like that! (I'm originally from Pedro
I'd try some Greased Lightning, and then I'd try some Zap, they both remove mildew, the Zaps has acid in it, so you need to rinse well.
Toilet bowl cleaner also does a fair job on mildew (acid), and it gels up nice so you can get your tooth brush in there.
You can also scour with Clorox grains (bleach) or Bar Keeper's Friend (acid).
If everything fails, you can try some Mary Kate and Ashley's On/Off which is just freaking unadulterated acid. It is a hull cleaner, so it may be a little rough for the vinyl. Have a waterhose standing by...