Andy, if your van has the Al-Ko type stabiliser, with the friction pads built into the hitch head, then you have to clean the paint off the towball to make sure you do not contaminate the pads, and render them useless for their job.
It is also important each time you hitch up to ensure the ball is free of rust or oil, usually a wipe over with wet or dry , or a nylon pan scourer followed by a clean with brake cleaner is all that is needed.
Never apply grease to the towball if you have the friction hitch coupling.