Dirty sealant

If you Google the question you will find lots of suggestions from others before you - they include white spirit, neat and diluted bleach, various proprietary mould cleaners, and usually involve someone else's toothbrush. I never bothered, so have no idea if they work or are safe on your paintwork. Try an unobtrusive corner. 

I use Silky Caravan Cleaner.

As above. You can also use that toothbrush with some toothpaste of the bleach version of Jif/Cif cream cleaner. I think it's the blue bottle not the yellow one. Make sure you are not to aggressive on the paintwork. I'd apply some car polish/wax when finished to give a bit of protection.

Colin