We have been using absolutely no additive to black water for many years. Works for us.
...this is a goal for most but, you know, we're too afraid of using too much water; so, we HAVE to add something or it stinks. I know, I know, just use more water and the problem will go away like magic; but, no matter how much I beg my lovely wife to use more water, she's afraid of fillin' our tiny-tank too quickly.
...just saying we all may have smaller spaces that don't allow the luxury of adequate water use; therefore, treatment is a necessity.
Cheap laundry soap, water softener, and bleach works like lots and lots of plain-ole water.
I don't use a lot of water either, but still do not need to use chemicals or additives. If your trap is working properly, you won't get odors in the TT.
Thank you for all for your responses and I'll look into each of those products. Actually, if my adult son with developmental disabilities wasn't so "addicted" to the cassette toilet not wanting to use the other facilities at all, I would probably not need the additive. Actually, he was almost impossible to toilet-train but when he was 6 years old, we went camping in a pop-up with a portable toilet and he started using that and never looked back, well, had a toileting "issue" after. It was a miracle worker! I will, therefore, always have a soft spot for portable toilets. I have learned many things from my son with developmental disabilities.