I found this info buried in an RV'ers blog the other day...Haven't tried it yet, but it sounds plausible...
Try this method of Gray & Black tank sensor cleaning when your rig is going to be at rest or in storage for about a month in ABOVE freezing weather:
1. Empty both holding tanks.
2. Add about 1/2 cup of liquid Septic Tank Enzyme Additive, the type that's safe for plastic tanks - to each holding tank.
3. Fill each tank about 1/2 full with fresh water.
4. Take a drive around the neighborhood to get the 'brew' well mixed.
5. Return to your storage area and add enough water to fill each tank completely.
6. Let it sit for a month to allow the enzymes to work on all the 'crud' inside, before flushing with fresh water.
If anyone's used this method, let us know how you felt it worked out...