I have used LA Totally Awesome cleaner and it is awesome.
All the "Dollar stores" and all big box stores sell it.
I use a garden hand pump sprayer and dilute it 2 cups water to 1 cup of product. Extend the awning, spray the underside (be sure to wear eye protection), roll up the awning for 5 or 10 minutes, extend awning and hose off the top and underside. You may need to do apply more than once and may have to hand scrub the spots that are bad. Be sure to wet the side of your rig before and rinse well after. I have a friend who does not dilute at all.