Many RV builders added the primer circuit before Onan actually added
it from what I can determine. It is a function that can be installed on any
vintage Onan with an electric fuel pump. II have had it on every RV
equipped with an Onan from 1983 models through my Class A that was a 2004.
An internet search turns up several hand drawn circuits for adding
the function and the same questions that have been asked here have been
asked several times before on other sites with no one well defined answer.
So it seems as if Onan decided it was a good idea at some point.
Individuals and RV builders were adding it in a couple of ways before this.
Some of the circuits consisted of adding a momentary push button to the
fuel pump often drilling a hole in the Remote switch panel to add this.
Here is one article about it The section labeled 15 shows how it was added.
Could be added to those that don't have it now probably.
http://www.gmcws.org/Tech/dsimmons/onan/onan.html