There are many sources of black tank odor. Could you be more specific?
Does it smell bad when you open the camper with empty tanks?
This might actually be from your gray tank venting through a dry trap in the shower or sink.
Does it smell bad all the time or just when your tank is full (or partially full)?
A treatment with RV-Digest-it; followed by the geo method should solve caked solids issues.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW