If climbing onto the roof isn't a problem for you, this is how I do it: get up there with a hose, a bucket of bleach and water (I go about 25/75), and an expandable brush from Home Depot. I rinse off the awning and then soak it with the bleach/water mixture (careful to keep off the roof) and roll it up. After 5-10 minutes, I open it up again and rinse the awning off. Sometimes 2 or 3 applications, but it comes out nice and clean. Then I go underneath and hit the spots with the brush that didn't come clean. It takes awhile, but I don't mind. Having a dirty awning, especially on the bottom, is something I notice all the time. This method is close to what is recommended by the awning manufacturer.
2010 Montana 3455SA
Mor/Ride kingpin, wet bolts, Progressive Ind. HW-50C EMS,TST 507RV TPMS, GY G614's
'07.5 Silverado D/A LMM 3500HD
Extended Cab DRW 4x4 LTZ
Bilstein, Reese R20, Banks Economind tuner, Speedbrake, RamAir, Monster exhaust