Is everything working? That is 12 volt
lights, fans, furnace, water heater and refrig controls? If so, it is probably an extra slot. I would see no harm in inserting a fuse in the empty place, probably a 15 amp one. If it is unused nothing will happen. If it is connected to something, that should work now. If there is a problem, the fuse will blow and you can have it serviced.
I don't know your rig, obviously, but those comments would go with any Airstream fuse slot that you can't identify in any Airstream.