Originally Posted by dumboat
Probably silly question, but can you pour rv antifreeze into the grey/black water tanks to keep from freezing?
Not a silly question at all. Yes you can add antifreeze to the tanks. It doesn't need to be a true 50% mixture. Less mixture will keep it from freezing solid. Slushy is OK.
For the most part a heel of water laying in a poly tank isn't a real problem. It is where the water is contained in a small area that the issues begin (dump lines, other fittings). A single freeze isn't usually fatal. Freeze thaw cycles are what do the most damage. My explanation of the mechanics are this. The water freezes and expands a little bit. Then it partially thaws (a solid plug of ice remains), more water moves in to the voids, and then it freezes again. Each cycle adds more expansion until something finally breaks. That cycle is obvious in old copper tubing in summer cottages when pipes don't drain completely. In the low points you often see pregnant areas in the tubing. Copper will tolerate some expansion without failing. I think that people would be surprised to see just how pregnant the copper tubing can get before finally failing. Hard plastic fittings... not so much.
I know of people who use RV's in the winter. They leave the fresh water system winterized. Many use a container of water/anti-freeze solution for manually flushing the toilet. I find that most times peanut butter jars are a convenient size for one flush. Some people use a jug.
Were I to use the camper in winter (we're considering that so we get some more use out of it) then my mode will be to fill my jars with 50% antifreeze solution. I am not speaking from experience for winter use, just what I've learned working on plumbing systems, and what I've gleaned from the forums.
Edit: P.S. - Even though it is going into wastewater tanks, please still use the pink RV type anti-freeze. The automotive ethylene glycol is just so much more toxic.