I had the opportunity to speak with a technical support representative at the Jayco factory today.
1. IF your model trailer DOES have a gravity fill inlet port then the FW tank can ONLY be filled via the gravity fill inlet and may NOT be filled via the city water inlet source even though the manual does say gravity OR city-water.
2. IF the trailer does NOT come fitted with a gravity fill inlet then the FW tank may ONLY be filled via the city water inlet. (obviously)
So it's one or the other (depending) but not both.
The owners manual could be clarified to avoid any confusion for the new owner.