Originally Posted by tafische
It is not the freezing that is the problem - it is the thawing!
So another possible option - but not a great one -- so it sounds like there is somewhere around 10 gallons in there. How much antifreeze would it require to protect this -- I am thinking quite a bit. Does antifreeze tend to sink and go down around the valves, float on top, or mix 50/50? It would be pretty expensive, but cheaper than the damage caused by freezing.
Assuming you're going to use RV antifreeze, there are two things to consider:
One- RV antifreeze works differently than the automotive variety- it does
actually freeze, but doesn't expand so your pipes won't burst. Just ask any RVer that has a refrigerator with an icemaker about making the pink ice cubes when they winterize.
Two: You use RV antifreeze undiluted so I'm not sure how effective it will be mixing it with the current contents of the tank. And since the trailer's not going to be moved, how do you ensure it gets mixed thoroughly.
As for using an automotive antifreeze, again, you'd have to figure how much to use to get it 50/50 and how will you mix it. Also, you'd have to find out what damage, if any, automotive antifreeze might cause to your tank, pipes, valves and seals.