There's a product that a lot of RVers like named RV Digest It
. I've never used it but have read some good reviews about it. I have used the Geo Method
in the past and it is a good way to clean and deodorize your black tank.
We've been traveling full-time for 9 months and in our 36FL for 5 months and haven't had a black tank odor issue while we're camped. If we do in the future, I'll use the Geo Method.
Some things to remember about the black tank is that it is a holding tank, not a septic tank so solids aren't usually in it long enough to really break down. I would think chemical additives do little more than act as a deodorizer. Just my opinion.
Another thing...don't empty the black tank until it is almost full
... I've seen people at full hookup sites with all their drain valves, including the black water tank, in the open position. It's ok to do this on the gray but not for the black tank. You need a lot
of water to move that stuff out.
Also, if your exhaust fan is running, especially if it's a Fantastic fan, when you flush the toilet you may get some odor. The fan moves a lot of air and the gap at the bottom of the door may not be large enough to provide it with an adequate air supply so when you open the toilet valve it will pull air up from the black tank. We've even noticed some odor in the bathroom when we move to a new campground if we forgot to close the bathroom vent.
That's all I have for now... sorry if it was wordy.