Bleach and water.
I make up a warm water bleach mix of about an oz bleach to 1 gal water.
I have a pump up garden sprayer I use to soak down the white underside of the awning. After its totally wet I'll roll it up and let it soak for 20 minutes or more.
Then I'll unroll it and with a car wash brush, brush down the underside, a good rinse and it's bright white again.
2015 Jayflight SLX 287BHSW
2002 F-250 Extended Cab 4x4 6.8
Teconsha Prodigy P3 Brake Controller
Reese SC Weight Distrubition with sway control (12,000lb)