Originally Posted by markopolo
Consider "flushing" the toilet using a RV antifreeze so you don't dilute the rv antifreeze in the black tank too much. I'd also add more rv antifreeze to the gray tank periodically so it doesn't get too diluted.
Right on Marko. Your Grey and Black tanks have lots of room for ice expansion, but your plumbing to your dump valve doesn't. (don't know the properties of RV AntiFreeze, but unless it's heavier than water) So keep lots of Antifreeze in the tanks.
You can get heaters strapped to the bottom of the tanks, but the plumbing to the dump valve can still be an issue.