A good working sewer system normally has a gradient of 2%, less than that and the fecal matter and paper sotp, more than 2% the water carring the matter tends to run off leaving the matter behind.
The black water tank may or may not have a gradient built into it, so you flush the toilet and the matter hits the bottom and the water splashes away from the clumps, some matter remains some moves a short distance and the water runs off,
now the fecal matter starts drying in place slowlly and eventually will become hard as concrete, no repair shop will want to clean this mess when the toilet can no longer be used, they will replace the tank, with a higher labour cost.
Leaving the valve closed and dumping when it is 3/4 full is recomended so that most of the fecal matter and toilet paper get flushed out of the tank, and even then there will be some matter left, thats when you get a cleaning wand attached to a water hose, a back flush adapter or you install a tank fushing gismo, calculate how much water you are dumping into the toilet so that you more or less know when to dump, there are chemicals that help control odor and start decomposition of the fecal matter.
Some tank chemicals have formaldahide in them and that chemical is now being banned in some states and RV parks with septic tanks, remember the jars with specimens in biology class in high school, preserved in formaldahide, unles you want to preserve you crapp I do not recomend those chemicals!
navegator