I agree the Winterhoff is a very good unit - mine has been in use for about 14 years on 2 different vans. They (used to) come with sets of shims for different drawtube diameters, but these are not needed for caravans. The ws 3000 is good for a hitchload of 150Kgs too if the towbar and car are up to that.
In use you have to release either forward or backward pressure on the hitch, so that the bellows is neither stretched nor compressed, or it is reluctant to release