The cheap level sensors used by the industry do not work well in the black tank. Use lots of liquid in the tank and only dump when you have over 1/2 tank of black water.
About twice a year, I will dump the tank and refill to about 1/3 full and put about 1 cup of dishwasher detergent in the tank right before we leave. Up on arrival, I immediately dump the tank and then use the flush to finish clearing the tank.
Some are obsessed about keeping the black tank clean enough to eat from. It is a holding tank for poo. You will not get a bacteris colony established because it is not a septic system. So lots of liquid and flush occasionally.
Ken