Unroll the awning and release the bottom, let it slide under the camper, go about half way with one side then go to the other so it doesn't get twisted. I let it all the way down flat against the camper to wash the top side then raise it back up to do the under side. The vertical braces will have to come apart to let it all the way down, they can be a little trouble to get back together, might take two people to do it.
I use about a 1/4 cup of car wash, 1/4 cup of bleach and a couple of gallons of water. Wash a section down good, then take a Magic Eraser and scrub it. Rinse it and start another section. The top side is easier than the under side and you get plenty wet doing the under side. You'll also need a 6 or 8' step ladder.
The car wash soap gets the dirt, the bleach kills the mildew and the Magic Eraser gets all kind of stuff off without too much elbow grease (I ain't sure how they work but they do).