Try mixing a solution of chlorine bleach(20-25%) and water in a spray bottle. I used to have a bad mildew problem when I lived up north with my boats. Mold and mildew over the winter. I tried everything and this worked the best. After cleaning I followed up with a good vinyl treatment and UV protectant. The bleach brings out the color and whites of the vinyl. Just make sure you rinse it good immediately after cleaning. I use this anytime I get mildew on life jackets or outside furniture.