I have had a similar problem on the plastic trim around the windows on the doors, this is hard plastic. the cleaner that i use is called " EXITS " I use it full strength for this type of problems and I use a brush, It works pretty good. My RV dealer carries this product. The problem with white plastic trim is that it is not ultraviolet resistant, it well break down due to the sun and the elements, but yet black plastic is ultraviolet resistant. Example: wire or cable ties, white or clear ties should be used inside and black can be used inside or out side. If you buy a pack of black ties, it will say ultraviolet resistant for out door use.