We use black tank treatments and water softener. We have used soap too.
We haven't dry camped much and always connect to sewer at RV Parks.
We let it get pretty full before filling draining and flushing.
Just used the soap and water softener a few times as thought it a good idea.
Used the ice cube, soap and softener idea too and drove a long ways then flushed the system. did we need to I don't know only once or twice have I thought we had a problem. RV Parks with lots of water pressure really clean the tanks with the factory sanitizing system.