Truth is because none of us have any experience with the type and make of motorhome you have, Australian right-hand drive, we can't really give you an exact answer.
For just water stains I have had very good results with some spackle to smooth it out and good quality semi-gloss paint. For major damage that would require replacement, the framework is important. If it's made out of wood that might need replacement too, If the structure is in good condition, you might be able to remove the damaged panels and put in new wall board or paneling. Most of these rigs have a pattern on the walls so painting throughout is probably required to make everything match. Look for the closest texture to your existing wall. Good luck
Linda S