I used Auto glym Caravan & motor home cleaner. I was really suprised how much more grime it removed even after just washing it first with a good quality wash & wax. .
You spray it on, then lightly work it over the area with a clean damp cloth then leave it for a minute or so, then with a sponge or cloth give the panel a good scrub as you do normally when washing a car or van then hose it off.
You then will need to polish it.