I have tried many many products on our caravans and found the easiest and cheapest method is car shampoo - wash the caravan the same as the car with bucket and sponge - however, when the black, ground-in stubborn marks appear, simply use a soft cloth and apply the car shampoo "neat" and rub gently - keeps caravan sparkling new! Any "extra" stubborn marks and I use a little polish called "Mer" (its in a blue container and sold in caravan dealers and good car shops) again, rub on gently with a soft cloth then wash off with the car shampoo! Easy, cheap and the best results - protects the caravan with that "new look" for years!!