Won't Clean

We have Winn MH, the style with decals (not full body paint - I wish...)

No matter what I try to use to clean I can't get grey grunge to clean away from white fiberglass areas, it's like it is under the clear coat. Its the side that gets the full sun exposure if that has anything to do with it. Other side is fine.

So far I have tried:
- Bug & Tar remover
- Bartender's Friend
- Black Streak Remover
- Color Back
- RV wash/wax

Any suggestions?