I bought Happer Camper deodorizer https://amzn.to/2CUHkfm for my black water tank (and also gray water tank) because it got good reviews.  I know the black water tank is about 9 gallons, the gray water about 21 gallons.  The instructions say 1 scoop will treat 40 gallons.  Since my tank is so small I figured 1/4 cup for the black water tank, but I called the manufacturer to be sure.  After a lengthy conversation, the gist of it is use 1 full scoop in 2 - 3 gallons of water each time you are setting up (after you've emptied the tank).  That means 1/3 of my tank is already used up and I'll have to dump more often!

So, I'm looking for other deodorizing options that don't require so much water to work.  Anybody have something they like and is easy?

BTW, I have no sense of smell so I'm doing this to make sure others don't think I'm just a stinky camper!