I usually just go to Dollar General or some other type of discount store and buy rubbing alcohol with the highest alcohol content I can find. It usually doesn't take that much to keep the fluid from freezing. It also helps if the trailer brakes freeze. Pour a little in the air lines, pump the brakes and use all the air devices in the truck and everything will unfreeze. You can often find a bottle for about $0.59. I prefer putting some Dawn in water for my windshield fluid and adding alcohol. It works better and costs less than the premixed fluid available at most retailers.