We were looking at our awning that has a little mildew. Someone at a campground told me that he uses a 50% mixture of Simple Green in a spray bottle. He then roles the awning up for 30 minutes. After ...
Once clean I allow mine to dry thoroughly and then lightly sprinkle Gold Bond medicated powder before rolling up. I even sprinkle some on a brush and wipe the underside.
The Gold Bond has zinc oxide in the mix - it is a great mildewcide... Used for years in paint to control mildew. I apply a new dusting before storing the trailer after each trip.
I actually buy the cheap medicated powder at Walmart - just make sure that it contains zinc oxide.