What makes you think you need ANY chemical in your black tank? If you follow the correct procedures in dumping and rinsing your tanks and all seals/gaskets/vents are in good operating condition, you should not be experiencing any odors at all.
Do you have an in-tank rinser? Most rv's today come factory equipped with one. If you don't have one, purchase and install one. Using plenty of water every time you flush the toilet will also insure you have lots of water in the tank that will enhance the flow to allow all solids to drain properly.
I also use the water in my gray tank to help in this process too. I installed a 3rd drain gate at the bottom of the "Y" where my gray/black drains are connected. Once I drain the black, I close the 3rd gate and open the gray gate to allow half the soapy water in the gray to flow into the black. Once that is done, I close the gray gate and open the 3rd gate again to drain the black tank once more. You'd be surprised at the amount of stuff that was left in the tank after the first dump.
You don't need chemicals...they only provide $$ revenue for the mfg.
Ron