I followed every recommendation I ever saw on here and nothing ever came close to working. Now I take it to the truck stop and have them acid wash it once or twice a year and it looks brand new. They charge $15 plus the price of a wash so about $25 total if I remember correctly.
Go to Home Depot, or someplace similar, and buy the Citrus clean stuff. Works well and it's cheap.
The other way to get rid of black streaks is to buy awning cleaner, wet the trailer, spray that on, let it sit, then wash with any carwash.