I went to the Flush King Web page (flushking.com) and read about the device. I think I'll try to find one, as it will accomplish a lot more than the item I originally described.
That is: The flush device that I described above had a knife valve that attached directly to the RV's flush-out port. That valve was connected to a clear tube with a flush-out hose fitting.
On the other hand, the Flush King has a clear tube (with attached flush-out hose port) that attaches to the RV's dump port...and the knife valve is at the end where the sewer hose connects. That way I could close the knife valve and then leave both of the RV's Gray valves open, which would allow the two tanks to equalize and provide the full 80 gals available. When dumping, I could then momentarily close the two Gray valves, then dump the Black tank, followed by a flush with water from the combined Gray tanks. By closing the Flush King valve, one could then flush out the individual RV tanks by proper manipulation of the respective valves.
I have a factory-installed Black tank flush-out system. But I DO like the idea of being able to also use the Flush King to do a clean out of the Gray tanks.
Thanks for the tip, Ducky....this forum is GREAT!
'14 Coachmen SportsCoach Cross Country 404RB
'15 Jeep Wrangler Unlimited--TOAD