What To Use To Clean The Outside Of A Trailer How Do I Clean Black Stains From The Sides Of A Cargo Trailer?

How do I clean black stains from the sides of a cargo trailer? - what to use to clean the outside of a trailer

It seems that was a long time ago, my trailer custom black paint ugly now, traces of water flowing along the side walls. Simple Green / Purple Power / car wax have little effect if the muscle energy is used by small area. Does anyone know an easier way to remove these ugly stains?