I added a third, Valterra, bayonet gate valve (C) to the outlet of the wye where the black (B) and gray (G) tank drains converge.
I keep all three valves closed until dumping time.
At that time, open C and B and drain the black tank. When it has mostly stopped draining, close C and open G. That backflushes the black tank with a slug of gray water and loosens up a lot of other "stuff". Listen, and you can hear when the water stops moving as the tanks equalize.
Then, close G and open C and flush the backwash out of the black tank.
Repeat until the backwash runs, uh, clearish (grayish?), or until you run out of gray water.
Open all 3 to drain out whatevr is left, close all of them, disconnect, and clean up.
A Rhino or other drain hose with a clear elbow at the far end makes this a lot more easy to inspect the flow.
I first built something like this on our 88 Suncruiser from parts before the cool Valterra "repair" valve was a thing.
Here's a link to the valve:
https://www.amazon.com/Valterra-T58-Twist-On-Waste-Valve/dp/B000BGHYJS/