Stability towing Lunar Delta r i

Sorry if anyone has seen this post in other parts, but relatively new to the pc.  Anyone out there tow with a lunar delta r.i?  Weve a kia sorento 2.5, both times on m56 and m6 travelling have experienced severe snaking and unstabled van!!!  Now wondering if its the van with transverse bed layout being the issue, or is it the car???  Had car checked and serviced and passed a mot, but still having stability problems it starts at 49 mph.  Told dealer and it is booked in for its 1st service and check over but not till mid nov!!!  Weve had many twin axles before as been caravanning for over 30 years, never had issues with any towing before.  The hubby is an HGV driver so he knows when something is not right and we both are stressing over it all,  .  Do we have to change the car?  Does anybody have same outfit , to compare?