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...
I use Simple Green and a Mr. Clean magic eraser mop head on a 10' pole. Works great and is very easy. This was on a "vinyl" type of awning materiel not fabric.