Welcome Nicolas. The Feeling is fairy heavy for your car and I think you only have 105bhp/PS or 75Kw so you should be taking things slow and smooth.Do not expect to make quick progress and 85/90kph really is your maximum speed for safety.
If we assume the caravan is in order perfect and the hitch damper is working correctly, then my guess would be the nose weight of your Eriba. I would aim at 60kg as a starting point to experiment with. To measure it sit the van on smooth and level ground. Set a pair of bathroom scales on the floor underneath the hitch, preferably on a smooth piece of plywood/chipboard etc. Rest a lump of wood vertically on the scales and up into the caravan hitch. Slowly wind the jockey wheel until all of the weight is resting on the scales.

Colin