Gardena makes a water meter that will hook up the a hose. When I dump, I turn on the tank flush & leave it running while the tank is dumping. Helps to stir things up & get them moving down the drain.
Then I close the drain valve & let the tank fill to about 1/4 or 1/3 & drain again. After which I put about 5 gallons in the tank. It's nice knowing just how much water is going in & not worrying about creating a black water geyser out of the roof vent.
The tank flush quit working shortly after we bought the fifth wheel. I removed the nozzle, which had a tubular screen on the end that had plugged right up with fibrous gunk. I cut it off & fashioned a better nozzle from copper tubing. Been great since.