Be careful about using any abrasive substance as it may remove the clear coat if your coach is painted. It sort of sounds like the rear of your coach was not protected with a coat of wax so the mud was allowed to penetrate the surface.
I would not use anything that does not say that it is "clear coat" safe. Perhaps a medium clay bar would work with a minimal amount of effort. Just be sure to use the clay bar with plenty of water to lubricate and wash away the debris.
After the coach is clean, apply a good coat of wax to protect the surface.
