Originally Posted by leland1950
One thing I have never seen mentioned is the use of Rock Salt, the kind they put on the roads and in an old hand cranked ice cream maker. A dealer that we had at our FMCA meets swore by it and several members tried it and had good results. I never did and can't remember the formula, but a little research might come up with it. It would clean the tank (looking down the toilet) like new. A good flush would be mandatory afterwards.
From ehow (not sure if we can post external links):
Connect a dump hose to the dump system, and open the black water dump valve to drain the black tank. Close the valve when the dumping process is complete.
Add 3 pounds of salt to each gallon of water to be added to the black water tank. Typically, 1 to 2 gallons of water is used to winterize the tank. Ensure that all the salt is dissolved in the water, before you add the saltwater to the black water tank.
Add 1 to 2 gallons of salt-saturated water to the black water tank. Salt saturated water will lower the freezing point to approximately -6 degrees Fahrenheit (-21 degrees Celsius).
Add other chemicals required for black water tank maintenance. Aqua Kem, Camco, Exodor and QT RV Tank Treatment all provide products that deodorize, breakdown waste and reduce solids.
EDIT: Actually I think this is more for winterizing?