In addition to all of the good information included above here's another freebe to help control black tank odor. Use generous amounts of flush water. We have observed that when we are either dry camped or camped without a sewer hookup that we tend to use less flush water in order to go two weeks without emptying. Whereas when we are hooked to a sewer we use much more water to flush and usually need to empty in a week. The longer period without emptying has caused odor buildup thus teaching us that the greater use of flush water does help with odor control. We also keep the toilet very clean as this, too, can be an odor generator. For chemical assistance we have found the use of RV-Trine products to work exceptionally well.