As I mentioned yesterday in the four-year-old thread:
Our Safari is our first RV and when we received our dealer walk-through and orientation last January, we were given a complimentary trailer starter kit, which included some holding tank treatment samples. We were told that it is a good idea to add the treatment with a gallon or so of water to each tank. We then picked up a 68-oz. jug of the highly recommended Odorlos
(Non-formaldehyde, Holding Tank Treatment) from Camping World and have been using it after each dump of the tanks (black and gray).
It seems to be working just fine. We have never had any trouble with odors. And we liked the idea that this product breaks down waste/tissue and lubricates valves.
As others have said in the older thread, the value and use of chemicals depends on how often you use your trailer. Those "on the go", full-timers, etc., probably do not need to use chemicals. But for those whose trailer sits for awhile between trips, such as ours, it is probably wise to use, what 2airí would say, a top tier holding tank treatment product.