Stephen...We had a similar problem with our last van (an Elddis Odyssey) It was finally tracked down to a faultly Winterhoff stabaliser which was exchanged under warrenty. Our current van, an Abbey, is made by the Swift group and I think they all have AL-Ko stabalisers. My AL-KO has not given any trouble at all.
It is essential the towball is kept spotlessly clean and if at all possible dry because if not you will get a creaking or clunking sound. Best get your dealer to check the hitch out and maybe the stabaliser pads need adjusting.