This may sound like an odd question so I'll give a little background first. I'm a little OCD about flushing my tanks, ok ok, I'm A LOT OCD about flushing my tanks. I've been in a couple campers that...
I use the clear adapter shown and when the last of the black water stops running I fill the toilet bowl completely with water (about 2 gallons) and flush it, then repeat. This gives a four gallon flush after the tank has drained.
Add two more gallons and a packet of nice smelling enzymes and we are good to go.
It is a septic tank, it is not supposed to be sterile.