No one mentioned this, or at least I didn't see it. After you flush the toilet, put some water in the black tank. I fill the toilet bowl 3 times, 3 gallons. You may want to use more or less depending on the size of your tank. That way it never dries up and you don't have to use as much water when you flush.
BTW,I stopped using chemicals years ago when I figured out that the salesman who recommended the chemicals was more interested in making the sale than in what I needed. Have never had a problem with the smell.