It really is your call, from our experience it never hurts to have a portable genny as well. Our RV has an Onan onboard genny that powers our entire rig as if plugged into shore power along with all the convenience of one button starting/stopping. That being said, we love our Onan but it is major overkill most of the time producing way more power than we actually need.
After nearly a decade of heavy usage from our very reliable Onan, I decided to get a portable inverter genny to save on fuel and reduce noise output. Main objective for portable genny is to run our rig's AC unit and still have the convenience of one button starting/stopping. We purchased a Champion 3100 with remote start and our Onan has been on vacation ever since.
