31 below zero, RV antifreeze failed?

Not to push where I work, but here is an online article that should help you understand the ratings a bit better. 
In a nutshell, the rating you use, ie -50, is what that version will burst a copper pipe at! BUT some will actually freeze at say 10-20F temp. So, what your gallons you have, have done, is normal! Froze, but have not expanded to burst your pipes!
The client I mentioned wanted the wintersafe, vs std -50. As you can see in the article, the wintersafe went solid before the std -50. Which is what the client found when he put some in their freezer! I work in the wholesale side of things. We sell 300-400 55 gal drums to various boat dealers getting them prepped for winter use. Ranger Tug buys this stuff to put in waterlines for boats leaving the area to other places from October to March. We ship 10-15 pallets with 4 drums per to a place in Alaska too.  
If you are near the WM in Spokane, they should be stocking the -50 or -100. Or it can be shipped to you next fall. The minus 100 might be a better option based on temps you are having from an additional safety standpoint. Our brand is made by Starbright, you might find places with that brand, or another companies version, made by starbright cheaper if you hunt around. At $5-8 per gal with a quick online search, shopping at multiple places will not save you a lot per say, if you only need a few gallons. A 55 gal drum might be another issue at $350-400+ per barrel! 
Here is a link to article, it's time to go to bed, did not realize it is so early in the AM! LOL

How to Choose Antifreeze for Your Boat Engine | West Marine

Marty