I actually saw one of these rigs at an RV shows in the mid to late seventies. You would be very familiar with your travel mates. I want to think they had a larger unit on a Buick Electra 225 as well. Obviously the idea did not take hold. Did you notice the mount was actually a regular tounge and ball, not a king pin?
2011 Ram Truck 2500, Crew Cab, 6.7L CTD, 4X4, 3.73 Rear