The term “fifth wheeler” comes from the fact that the modern day fifth wheel hitch, resembles the horizontal wheel-like bearing that enabled the front axle of a four wheel horse drawn wagon to be steered. This steering bearing was in the first instance built like a wheel on its side. Hence “fifth wheel”.
As applied in the recreation vehicle industry the arrangement is simply a smaller version of the system which has been in use on every semi-trailer for decades.
In North America rigs using this type of hitch are the most popular. So the concept is not new.