Are you sure it is the black tank?
Under the bathroom sink there ay be a vent for the sink. It is common. The vent is essentially a one way valve allowing air in but shutting when not in use. The vent sometimes hangs and smell come up from the grey tank.
If you are sure the black tank is the culprit go to a pool supply store and get "SHOCK "(sodium dichlor). One purpose of shock is to breakdown organic waste contaminants. Add water to at least 3/4 full and drive around. let sit in tank for a day or two then drain. Problem should be solved