morning smell

If you're dumping every three days it's highly likely you're leaving a LOT of solids around since in three days you wouldn't be building up enough liquids to flush solids from the tank. That is assuming that the smell is from the black tank but often gray tank odors are almost as bad a black tank, esecially if your gray tank contains cooking waste. With a separate galley tank the gray stays fresher. It's ALSO possible that the odor is from your battery bank. Sulfuric acid smells a lot like the black tank. Check to be sure you don't have a battery that's getting hot or bulging.

Good luck / Skip