Won't Clean

Sounds like what our '03 Adventurer looked like, we purchased it used. Sides had gray/black imbedded in the fiberglass. Tried various cleaners without much success. I finally tried Zep mold/mildew stain remover. I was reluctant to try because it contains bleach but it worked great and didn't harm the decals or painted compartment doors. Spray it on and watch the black and brown goo run down the side then rinse and rinse. I started with a small spray bottle then had to buy a gallon jug to refill the spray bottle. I followed that with nano wax, doesn't give a super high gloss shine but definitely makes it look better. I used Protect All on some sections and Turtle Wax spray nano wax on others. I'll see which holds up best. No hard buffing with either one and both have UV blockers. Rub it on then wipe off with a dry cloth. Doesn't look like a coach with full paint but sure looks a whole lot better.