White King at Albertsons.
I too could not find Calgon. I have a 70 gallon black tank.
1 cup of laundry detergent and 1 cup of water softner down the toilet. Do it in a flushed out tank with 5 gallons of water in it before you go down the road and you will find it very easy to flush out the black tank. It keeps the "stuff" in suspension rather than stuck to the tank. It washes off what is stuck to the tank walls. Human solid waste is very greasy and this overcomes the grease.
Ralph & Snickers
2006 3500 Chevy Dually - 8.1 - Allison
2006 30' New Horizon - Solar