Before joining the RV.Net Forums, I knew nothing about motorized RV's other than I wanted one, now I know a little more and one thing I have never had a problem with keeping the tanks cleaned up and not smelling.(An aside, the AAV's were both bad when we bought the Winnie, repaired and no more sewer gas smell in the coach)
I read lots of stuff here in the forums and decided to try some things for a problem that only marginally existed. The well publicized pyramid of poop did need addressing but it was hard to educate the DGKs. Now that they are more sophisticated in RVing, it's no problem.
I tried some of the chemicals, ICE w/o softener and others, the best thing so far, as repeated over and over, water and plenty of it, both during and after using the john. We do have a whirling spray flush system in the black tank and it takes far less time and water to flush than the wand or the back flush elbow methods. It is well worth the cost and when we get another rig that will be added if not already installed.
I do use a garbage disposal macerator pump at home and it is almost as fast with that and the flush system and it is with the 3" stinky slinky and flush system. I do like dumping right after traveling because as stated, everything is already mixed up and not settled on the bottom of the tank.
Chemicals, twice a year before storage, just a cap-full, no more. Preventative and probably unneeded.