2023 320 CS-S Grey Water Flush Input?

I'm trying to figure out how one flushes the grey water tank in our new 320 CS. The Nautilus P3 unit we have doesn't have a hose fitting in the Tank Flush location, and all the Nautilus instructions I've seen tell me I should be hooking up city water to this input to flush the tanks, but assume this is the black water or combined black/grey water tank. We have the cassette black water tank, and I expect that's changed how the system works. 



Seems like we'd still want to flush the grey water talk occasionally, no? Is there a way to do so that's better than simply running city water through the taps and down the drain with the grey water dump valve open? 

Thanks!