According to the FMCA historian the term "fifth wheel" has a practical origin that dates way back like to the 20's or 30's. At that time spare tire/wheel assemblies were sometimes mounted laying down on the trunk deck, and referred to as the fifth wheel. The inventor supposedly rigged a hitch in that position and thus was born the fifth wheel hitch. P.S. I've been camping for 20 years with owners of fifth wheel trailers and have not to this day heard them refer to them as fivers. They apparently have just stuck with the manufacturer's description. The first time I heard fiver used was by a host of an HGTV program about 5 years ago.
