Black mark, boy!

Greetings fellow vanners,

Can anybody advise how to remove the dark streaks that are beginning to mark the side of my van. They run down from joints in the frame which are obviously bonded with some sort of black mastic. I have tried everything; neat Fenwicks, Autoglym tar remover, Autoglym insect remover, Drywash for caravan; and while some of them have a marginal effect, the streak is still obvious.

Most grateful for your experiences.