Sounds like there may be two liquids in question here, the "flush" liquid and the liquid that goes in the cassette to aid in breakdown and odor reduction.
The best way to know for sure is to consult the manufacturer, especially regarding different models and whatnot, but here's what they basically say: flush with water, other chemicals can damage the lip seal. Don't leave water in the bowl, it does not reduce odor. And finally, they recommend Aqua Kem in the cassette, and so do I after using a competing product from West Marine.
1995 Toyota 4Runner SR5 4WD 5 spd
2002 Eagle Summit 12