If you have large amounts of fecal matter that hasn't been cleaned out you can flush with water or at some of the RV parks they have a service that comes around and will do a high pressure wash of the black tank.
I use TST black tank deodorizer available at Walmart in my tank. I know, I know, some people use nothing and say never a problem. Ya, but, I've been in their RVs you can smell the black tank. They just get used to it. Really doesn't cost that much more to use the chemicals that break everything down faster. PLUS, my tank monitors always operate correctly all the time in both the grey and black tank. Yup, I also use grey tank chemicals. JMHO.
