You will get a myriad of answers on that question. From magic water softeners concoctions to ice to doing nothing. :R
For years and years there have been holding tank chemicals for marine and RV holding tanks. They were made for a reason. Your holding tank is not your mommas house sewage system. We have small pipes, bends and blades and sensors on the inside. All places for stuff to stick and clog.
If you don't want garbage stuck all over the inside of your tank and sensors, smells when you flush coming up into your RV, and don't want to experience clogs in the pipes and valve blades to ruin a good camping trip use this. All the others just add 'perfume' to the tank and does nothing but smell up your RV with a nasty perfume smell.
Thetford makes 2 types one to appease the greenies and this one. The labels look similar. Be sure to read the label and it says breaks down waste and powerful odor control.
20 years on the road now and my sensors always work and I have never had one incidence of sewer smells in the MH zip zero.:W
We must be willing to get rid of the life we've planned,
so as to have the life that is waiting for us.