I think there's a misunderstanding about what @hud60 is asking:
does anyone know if I can pair a 2021 onan gen. prepped 5th wheel with a Honda 6500 generator? could a knowledgeable electrician be able to do this?
I read his question as meaning that he had a 5th wheel with an Onan generator "prep", but no actual generator, so he wants to know if he can make his Honda EU6500 work with it.
Others are assuming that he wants to run his Honda EU6500 in parallel with his built-in Onan generator (much like how some generators can use a
parallel kit to pair up 2 generators to generate more output). I think there's wide agreement that it is not possible to pair an Onan and Honda generator like this. (though if he has 50A service, he could conceivably run one generator on one 50A leg and the other on the other 50A leg, which gives mostly the same end result).
@hud60 will have to clarify which he meant.