We start the flush when we start packing up and keep an eye on the tank level, seems to get the tank clean.
Cliff,
Good advice with the fill and then dump, fill and then dump.
That's also what I do, using my watch to increase the time, first 60 seconds, then 90 seconds. Maybe a third time for another 90-120 seconds.
It allows water to rise pulling debris off the sides of the tank. My sensors usually work well also, with this method.
I don't walk away from the UDC with the black tank flush running however. I have just made it a point to stay there until I'm finished.
Sooner or later you are going to forget it is running and it can really mess up your day.
I have almost done this while standing there. Someone will walk up and start talking and after a minute, I remember. Oops!
Luckily I have always caught it so far, but I have talked to others who have either had water come up in the bathroom, or on the roof from the vent pipe. JMHO
Sorry, I got a little off topic from the OP.
I also use Cyclone vent covers on several of my roof vents. They really help with odors. Trace