The coldest temp I've observed here in 25 years was 14 deg F. My freezer is consistently between 0 and -3 deg F so I figured that was plenty cold enough for my purposes. I tried the same experiment you did, ending with straight RV anti-freeze. In all cases the RV anti-freeze turned to slush, but never froze solid. After studying the label, I found it's only claim was it wouldn't break pipes. So I concluded that whatever they used for anti-freeze stayed "loose" enough to push through the pipes (in case you wanted to use the vehicle during the winter), but wouldn't freeze solid and expand which is what causes the pipes to break. Not knowing if the crystals formed in the slush were sharp enough to damage tires, and considering the cost, I decided to pass. I got some WWF (can't remember if it was -40 or -10) and performed the same test. About 30% WWF kept the mix from freezing in my freezer, so that's what I used, cutting cost some over straight WWF even, and a *little* heavier than straight WWF. It works for me.