[CaptnJohn]

The black tank flush is a "feature" recently available. I had units since the 70s and many had not even thought of such a thing.
There were few problems if people used a lot of water in the toilet and didn't empty until over 1/2 full or heard the "burp".
At least one chemical supplier and many RVers say don't flush it out as then you flush all the good bacteria you have built up.
Me?? Now I flush in the spring when it's in storage for 3 - 3 months.