I use Tilex (bathroom mold cleaner). A good awning product would do the same thing (mold is usually the stubborn dirt on those, too).
Be careful with the Tilex...it will strip the wax right off the trailer's walls. I spray it on an old cloth and rub it into the handle...hose/rinse off with whatever is handy (garden hose, sprinkling can, etc.), you don't want to leave it on there to dry.
My 2 cents, your mileage may vary...
2 Cats; J-Lo and Ragamuffin :R
2014 Thor Tuscany 40RX DP
2011 Ram 2500 Longhorn CTD HO
2011 Keystone Cougar 318SAB (now gone)
2008 FunFinder X 210WBS (Sadly gone)