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.
BECAUSE CHIT stinks that's why and the good additives break down solids(drains better and cleaner) and help keep the odor to a minimum
lots of bathrooms have fantastic vents and most are open.Flush the toilet and the fumes get drawn in is one reason you want tank deodorant. Also a known fact is the BT vents out the roof,when its got a fair amount in it you can quit often get a down pressure or wind and it will stink up the campsite just a couple of many reasons