I used to use Turtle Wax shampoo, then give it a good polish with Mer on the aluminium & Turtle Wax polish on the GRP as it's not so coarse as Mer.
I now wash it with Fenwicks range of caravan cleaners, although more expensive, they work really well. I got some cheap from Halfords last winter.
Although it's a shamne to put the van in storage after all that cleaning the payback in early spring is that all the winter grime washes off really easily including the black streaks.
Happy cleaning!