I just ordered one of these devices. The only time we have noticed a black tank odor is when the rig sits unused for a while.
It usually goes away after we start using it again, but I can see where some would start overloading chemicals in a panic to stop the smell.
What I would like to see at the very least would be good ventillation going down the road so when we arrive at our destination we don't get greeted my that wonderful aroma when setting up.
EDIT - spelling corrections...
327RKS