These things here.
They are a whiz at getting sinks clean and removing lots of other stubborn stains, but they work so well because melamine foam is really quite abrasive.
It's like using fine wet and dry sandpaper, so if you were thinking about using a magic sponge for getting rid of black gutter streaks, I'd be inclined not to.
The body paintwork on these vans is absurdly thin - much, much thinner than car paintwork, for example, and there's no protective lacquer coat on it either. It takes next to nothing to wear it away.
And if you're ever tempted in desperation to try cellulose thinners to remove some particularly horrible residue that nothing else will shift, don't.
Before you can say Jack Robinson that shiny white paint will be on your rag instead of on the caravan, and you'll be staring at a patch of shiny aluminium instead...
.