I use a septic tank treatment (some sort of yeast I think) and a bit of Pinesol in the black tank. The grey tank can grow bugs that stink as bad as the black tank, so it gets a shot of the stuff also.
About every third dump, when the black is half full I add enough water to get it up near full, pull the dump valve while running the tank rinse. When the water runs clear I close the black and open the grey (grey is also dumped full.) and helps flush the hose clean. When traveling I keep some water in both tanks to help splash and wash things out, sort of speak.
We have a well and separate septic tank set up for my studio that I added a “dump station” to and about every three months I will let the tank flush run for 4 to 6 hours. People also do this at campgrounds letting the rinse run overnight.