Hi - I will add my comments but I am no expert on towing. All I do is be cautious and treat all other drivers as potential idiots.Clive perhaps you could help me with our towing problem in relevant forum?
One thing that helps with the larger lorrys is to move out slightly in your lane as they come up behind you then as they pass GENTLY move in a bit. This helps them move away from your lane and enables you to put as much distance as possible between you and "him"
I have never had a problem with smaller vans and the like. So sorry cannot help you there.
There is a guy who contributes regularly here called Lutz who is based in Germany and would probably have some insight - he is very knowledgeable - may be especially relevent with your make of 'van.
Why don't you flag it up on the other thread with a specific question ? I am sure Lutz would respond.
My only comment would be to look at the noseweight and how the 'van rides - is it nose up or nose down. Nose down helps!!
Also what was the road surface like? We had a terrible problem once with a folding camper that became sensitive to a sparrow breaking wind. What we thought it might have been was these "furrows" that get worn in the inside lane the track of the wheels on the trailer was such that given half a chance it would start to swing from side to side as it got caught in these tracks.
Hope this helps