The hose bib is on the driver's side near the hot water heater. On this model there is a second threaded cap near the rear, just like the 'drinking water only' cap up front, only it's not behind a locking flap.
Could it be connected to the black water tank... to be used for flushing it out? Do any of these models have seperate clean water tanks, one for drinking, one for showers, toilet, etc?