Laid Back (and all other members) I have removed the black streaks from my TT's over the last few years with a bottle of.....wait for it...."Black Streak Remover"
No kidding, I bought it at the local RV shop here in Ontario and it works awesome. I even used it on my new Laredo just today.
I believe the streaks can also come from the rubber roof run off as I have seen the streaks near gutters as well as near the windows as noted above.
I am sure your local RV shop will have something like this for you to try out. I will admit it does a good job considering mild soap and water did nothing.
Oh and Laid Back....welcome to the club.
I hope you like it as much as I like my Laredo.