Every black tank has a pyramid of some sort unless you have a bed of at least 10 gallons of water for poops to float in (after all, the tanks are shaped like brownie pans -- very shallow and the full width of the trailer. The tank walls are only a few inches high). Even if there is a lot of water in the tank to start with, if the poops land hard, they may stick.
We have a transparent plastic section on our sewer hose. When we dump our tanks, after running the fresh water flush until it runs clear we then pour a 5-gallon bucket of water down the toilet. This dislodges whatever kind of pyramid might have formed.
It always dislodges something. So we keep dumping 5-gallon buckets of water down the toilet until the water runs clean.
Usually 2 buckets does the trick. Sometimes a 3rd bucket is necessary.
It's the only way I know of to guarantee that the tank is actually totally clear of poop after dumping. A fresh water flush system just doesn't have the necessary force and doesn't spray water in the right places.
You may want to dump a lot of buckets of water down your tank to clean it out...
