RV Do
I don't often rave about a product - in fact I'm usually first in line to point out shortcomings with anything I buy.
Before we had our Heartland we had a fiberglass Terry and for years I tried everything under the sun to get rid of the black streaks. I was at an RV show in Raleigh, NC last year and there was a kiosk for a product called RV Do. I stopped to check it out and the lady told me that it was the only product that really works. I said something like, "Yea... I think I've heard that one before." She challenged me that if I'd by a bottle and it didn't work she would send me my money back w/o having to return the product. So I figured I'd give it a shot.
WOW! It actually worked! The older stains took some rubbing - and some didn't completely disappear. But the newer stains literally just wiped off.
I used it just two weekends ago on our Heartland before we took our first camping trip. Since we just got it in Oct of last year the streaks were pretty fresh. I just mixed some up about 3-1 with water in a spray bottle, sprayed and used a car/boat wash brush and wiped the stains off.
Here's a link to their web site. I very highly recommend this stuff.
http://www.rvdo.us/home
Really good stuff!
...Sam