Bike Racks

We brought a bike rack for our car several years ago with the intention of securing it under the car tow ball. The idea was to be able to take our bikes when we go away with our caravan. When we got the bike rack home we discovered a warning stick on the rack that advised against towing while the rack was attached under the tow ball. I have since been told by a caravan repairer that it is ok to tow while the bike rack is attached. Has anyone had an experiece with these bike racks and towing while the bike rack attached.