Black Tank Deodorant

Use more water. There is NO black tank treatment that has any smell I'd want in my bathroom. Using insufficient water is the more likely cause of your odor problems. Another thing you can do to assure no odors is to NEVER NEVER NEVER leave the black tank dry whether in use or in storage. There is always solids left behind no matter how much you flush your tank when dumping and these will turn to stone, create clogs and odors. I would never leave my black tank with less than 5 gallons of water in it, even when in storage.
Good luck / Skip