It was a 5th wheel built in the early 1970s by Harmon Industries called a Shadow that utilized a roof mounted hitch on an automobile. It was advertised as a fifth wheel that didn't require a truck to tow it. There were advertisements in RV magazines such as "Trailer Life" showing it being towed by a Cadillac automobile. It was never a success. There possibly could be one on exhibit in the RV Hall of Fame Museum.
Bill