Discovery 4 Towing issue

I purchased a new Land Rover Discovery 4 3litre V6 to pull a caravan estimated to be 3,200kg.

The power is unquestionable as it pulls it quite well, however the first trip my wife got motion sickness from all the bouncing around.

The vehicle is fine on most highways but as soon as the road gets a few undulations the vehicle travels like a small boat in a rough sea. Slowing down to less than 80kph helps but that can be dangeous to other vehicles. The ball weight is 260kg so that may be the problem.

Most of our travels so far the Discovery has been basically empty. Land Rover aren't convinced the problem is theirs and the caravan manufacturer doesn't believe the problem is in their caravan as it has been balanced correctly.

Can anyone direct me as to what I should do next in resolving the issue?

Brian, South Australia.