I almost bought into the chemicals thing, but after a year of practice I now dont see the need. I am not anal about my black tank (pun was intended), but I do believe that emptying the tank is not something that can be rushed.
I see other campers pull the valves, close em' up after a minute or two and drive off. Myself on the other hand, am spending some time using my Camco tank flusher to back fill my tanks a couple times and make sure everything is rinsed out. Yes, it takes a little bit of time, but I cant say that I haven't experienced any odors or issues by using the tried and true "use plenty of water" method.