Use a pressure washer to get rid of the bulky junk, mud, dirt etc. then spray Castrol Super Clean on it. let sit for about 5 minutes (do not get any on the paint, and do not let it dry) then hose it off with the pressure washer.
Then use one of the vynl protectant sprays to bring back the glossy black color you had when new.
This is what I do to my Line-X coated Rocker Pannels.