I have a 2015 Alpine 5th wheel. It was at the repair shop for 4 months after returning from a 3 month trip this winter in UT. When we brought the trailer home the awning which is white on the unders...
Clorox Clean-Up. Spray on a damp sponge/mop and wipe onto the awning. Wait a minute or two and rinse off. (I, too, tried a bunch of other options that didn't work.)