Swaying while towing

We recently upgraded to a 2.9 ton full caravan (River Titanium) from a 2.1 ton pop-top (Coromal 615S). We tow with a new Colorado 7 and use a heavy duty Hayman Reece Hitch. Everything goes well while towing except when a large truck, road train or semi overtakes.
When they reach the rear of the van there is a slight but frightening sway of the van. If I move further over to the left it gets a bit better, but it's still there.
Is this common for full vans while towing? Is there something I can do to prevent it, like fitting a different hitch or driving differently? Any advice would be nice.

Tom Miller, NSW