Rare indeed is the RV that has dual 12v & 120v light fixtures, and I don't know of any Fleetwood model that offers that. You didn't mention an RV year/make/model, but sight unseen I'm willing to bet the bulbs are all 12v.
Typically the switch is a two-way, so that it lights one bulb in one position and both bulbs in the other. Have you tried that?
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC