Your black tank is a holding tank. I'm not sure why people are trying to make it into a septic system. With all the money that people are throwing down their toilets with all these additives, it might be cheaper to get $10 worth of pennies and toss pennies in your black tank with a couple gallons of water while you drive home from the rv park. The pennies will probably do a better job of scrubbing the walls than all this other stuff that people put down there.