Welcome to the forum Marc and Renee. Glad to hear you are enjoying your "new" Laredo and that you have not experienced any problems with it. I am not sure if you will ever get rid of the black streaks as many of them originate from the blackened sealant around the windows and from the water dripping off the roof gutter end spouts. I am about to "install" a 3m product (EZE RV Windshield Gutter) over a couple of windows as a trial. It is advertised to "seal leaks around windows and prevents ugly streaking by channeling away dirty water". You can see the ad at
www.rvgutters.com This product is available at CW.
I think the key might be to find a good wax that makes the cleanup of the streaks an easier task.