We had some Black Spray paint on the back side of our camper , I was able to use a Citrus cleaner and a "Grout" sponge and got all the paint off without any damage to the original paint on the camper, Then, I applied Rubbing Compound to the entire camper, and buffed it using an electric buffer.... cleaned up nicely ...
you can also use WD-40 to remove graffiti ...