The hose less pit you describe. Sounds similiar to what commercial busses use to dump their tanks. I have never seen anything like this in a CG. Arte people just being lazy and inconsiderate and dumping without a hose?
If you camp frequently there is no need to obsess over cleaning the black tank.
Dump rinse and go. Trying to get it totally clean is unnecessary its a black tank.
Carrying stuff is its purpose in life. The tank will never be used for anything else. It will never be spotless no matter what you do!
Clean it enough not to build piles is all that is required.Excessive flushing is simply a waste of water and resources.