Totally agree with the folks telling you to keep the dump valve closed. The rig we have has been in the family since it was new back in January 1997 and there's never been a POOP Pyramid using the following methodology.
1. A couple of gallons of water in the tank before using it.
2. Only TP and human waste get flushed. (toilet chemicals are allowed)
3. Empty tank when almost full (if its time to leave campground we will fill tank via the tank flush system)
4. fully Open flush valve (leave flushing system running)
5. close flush valve (if there's time we will let flushing system 1/2 the tank and then open dump again)
6. If its warm season leave flush system run a couple of minute IOT leave a little layer of water on the bottom of tank.
7. Pull grey water dump valve to flush the hose.
Right now the rig is winterized when the toilet valve is held open I can clearly see the bottom of a CLEAN BlACK tank.