The best thing I found to clean a dirty, moldy, mildew, stained RV awning is 'Awning Cleaner' ! :B
Really I tried all the home made so called tricks and what WORKS is RV Awning Cleaner pure and simple. They know what stains RV awnings and formulated a cleaner for it. And more importantly they formulated a cleaner that won't harm RV awning material.