I have used a product available at Home Depot that really cleans up mildew and stains on vinyl siding. It is called JOMAX House Cleaner and Mildew Killer by ZINSSER. It is in the paint department. You mix it with water and bleach, spray it on with a pump-up sprayer and let it sit briefly, then rinse it off with a hose. Not sure if it would work in your situation, but it might be worth trying on a small area.
Gary