Ok talked to the Interstate folks again. Person actually said freezing point was 32 degrees bring them in. I questioned a little more and long story short had some doubts about the info. Called Camping World in Golden, CO, guy there says yes, bring them in rather than let them sit through the winter, but essentially if you're on the road and there's charge flowing in and out, there's no real reason to think they'll freeze or get damaged.
Should be said I have a 100 watt solar panel keeping charge at 100%, and was first assuming that would do the trick and I could let them stay out all winter (in full-sun uncovered RV parking).
Then came across this
. Says that as long as you're 100% charged you can go to -92 degrees! So it seems like, even with slight dips in charge at night, as long as you keep them at 100% the water in there will not freeze even at arctic temps. Great!