There is a product call Black Streak Remover you can get at most RV supply stores.
I also believe you can get it at most Meijers and Wal-Mart Stores.
I have heard some people use WD40; I personally have used Snow Bowl. The only thing I am unsure of with some of the personal remedies is do they harm the calk that is used to seal the unit with. What I can see it is the calk that causes the black streaks, so it only seem logical that some substances may remove the calk seal all together. The Black Streak Remover will cost about 4 something a bottle.
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