Why do you think you need a black tank additive???? If you have a odor, it's because you have a problem with your system. A seal may be out, vent tube blocked....etc. The black tank is sealed system (with the exception of the vent tube or when flushing the toilet) and should never emit an odor into the rv.
Black tank additives are NOT needed nor required. Much like special rv toilet paper, additives are just a sales gimmick for their manufactures. Using plenty of water during toilet flushing and when draining the tank should be sufficient. Also keeping the tank free of lingering "stuff" when draining will also help with odors and keeping the tank sensors in good working odors.
Best way for this is when getting ready to travel, drain and rinse the tank and add several gallons of water. Then put in a cup of liquid soap (laundry or dish) and allow the solution to slosh around when traveling. When you get to your destination, drain and rinse the tank once more.
Always keep plenty of water in the tank at all times.
Hope this helps
Ron