For years we have only used a 'scoop' full of laundry soap (any brand) and 1-2 cups vinegar in our black tank every time we dump, previously in our 5th wheel and currently in our motorhome. We have never had a build-up of any kind, nor any smell. Read it as a 'tip' long time ago as a good alternative to the expensive tablets etc. After dumping, we put some fresh water in the black tank, then add the soap and vinegar through the toilet inside.
