Dumping at Campgrounds and Flushing Black Water Tank Too

We normally flush when we have full hook ups and use dump station as that a "dump station".
Flushing as you said is not a good practice. Sure if you dry camp and need to flush, good you should do it when there is not a backup at the dump station.
Flushing through the toilet doesn't do as much good as connecting to an external flush system. Just filling the tank only removes debris that is not attached to the tank.