You are right, it isn’t so much getting your black tank “squeaky clean” as it is getting all the clumps & sludge out so that it doesn’t clog up and smell. My rig came equipped with a sani-flush system, a spray nozzle on the opposite end of the tank from the dump valve. It was clogged from the previous owner not maintaining or not using it. Had the occasional stinky days, used tank treatments. Been inoperative for a few years.
Finally had enough, pulled the nozzle out and drilled the jet holes 0.010” bigger and added a nozzle to the grey tank (didn’t drill the grey tank nozzle out bigger). Had to do some plumbing magic but hey, it works great. No need to sanitize the tank. Just dump the tank, turn on the flush nozzles, and the world smells nicer. Plus no complaints from the DW.