I would suggest your approach is incorrect. You will never succeed in making a gray or black tank contents "smell good" or "not smell".
Instead, concentrate in keeping the odor in the tank from getting into the interior. This is done by keeping air pressure in the tank lower than that inside the RV. There are several venturi-based devices that mount on the roof vent for the tank that help.
Also, check the air admittance valve under the sinks. Their rubber can dry out and allow gray tank odor into the interior.
Of course, there could be a physical reason for the change. For example, the vent pipe may have slipped down into the tank, a bird or insect nest may be blocking the roof vent, etc.