I am also in a 2015 Greyhawk. My set up is opposite yours, City water on top but otherwise I suspect is the same. As you noted one is the City water connection used for supplying water, filling a holding tank or winterizing the water lines.
The second will be your black water scrub, not to be misinterpretted as "sanitzation" which is actually cleaning your water lines. The second nozzle is connected directly to your Black tank and a series of sprayers inside the tank which clean out any debris left after draining the tank.
To use the black water scrub, ensure you have a seperate hose to ensure no possibility of contaminating your water lines. I recommend using a different colour hose then your water line to ensure no mistakes are made. Ensure your black water valve is open and connected to a sewer outlet and then run the water. You will be able to hear the spray nozzels flapping around inside the tank. You do not need to do this every time you drain the tank but considering what you are dealing with... more is always better then less.
Griswald One - Life is a Misadventure!
2015 Greyhawk 29ME / Chevy Sonic