Were it me, and if I had the VIN number, I'd call the factory and ask (OK, hope) that the wiring for a genset was installed at the time of the buildout, even though no genset was ordered.
Some manufacturers do, some don't.
If the wiring harness was installed at buildout, then adding a genset would be relatively straightforward. If not, well, things would get a bit more complicated.
Separately, as to Onan or Generac - though I'm partial to Onan - I'd install exactly what would have been installed at buildout if one were ordered at that time.
YMMV.
Cheers.