I use Fenwicks caravan cleaner to get them off. Before I park the van up for the winter I give it a good clean with Fenwicks caravan cleaner then spray it with Fenwicks overwintering solution. Come the spring a clean with Fenwicks Caravan cleaner at 1 to 10 gets its gleaming again and just for just good measure use Fenwicks Bobby Dazzler too.
It may sound like it but no, I do not work for or am an agent for Fenwicks. I 'discovered' it at a caravan dealers and have sworn by it ever since.
Just done our new van with proshine dry wash,it cleans and polishes in one application,easily removing the forementioned black streaks and also some tar spots when they were resurfacing a road we travelled on.You don't need any water with this product and we cleaned the van whilst on storage site.Thoroughly reccomend this product,forgot also did the windows and mirrors as it does not streak.Cheers John.
ps there's a video on you tube about this,sorry can't do the linky thing.
Just to reassure you, I think we all get black streak marks, irrespective of the vans age, so dont worry. Pain in the a** but there are plenty of cleaners to help you get rid.
------------- I started out with nothing and I've still got most of it left - Seasick Steve 2008