Had considerable problems getting a decent finish on my Hymer front end which is GRP. car waxes were OK to start with, but did not last, and often made it look dirtier.
Someone on this forum advised me to use the wax used on boats, which is a hard wax, which although it takes more elbowgrease to put on, seems to last a lot better. So for my van. it's 3M yacht wax !