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 ...
I use Lysol cleaner with bleach on mine - spray on and roll up and let sit for 15 - 20 minutes. Use the same and a brush to clean the outside when rolled up. Some of those stains are hard to get out. I was surprised at that since I keep things clean. I've also used scrubbing bubbles, and Weslesy's bleach white - the stuff for whitewalls and lettering on tires. Let me know if the Simple Green works for you - might be less harsh.