Have you tried Boat Cleaner Pro (formerly known as Marine Strip)? I purchased this to clean tuff spots on my vinyl and made it look brand new. Others have had mixed results but I'm glad I gave it a shot, great all around cleaner and safe for vinyl.
Twin 5.7L Mercs w/Alpha One Drives
Koehler KVA 4 Genset
My wife is a stain and spot removal Expert, her go-to is a bleach/water mixture, starting around 3/1, water to bleach. When that’s not good enough she starts cutting out the water. She got really bad mold spots out of our hanging locker below decks with a spray bottle of almost pure bleach, let it dry, did it again, and after drying the second time, the spots vacuumed out.