That was a close call!
The charge controllers will throttle down the current when the batteries seem full, as you assumed. However, the way the CC figures out that they're full is simply by tracking the trend of current demanded to maintain a given voltage, and holding the voltage (this is for lead batteries, different for NiCad and Lithium, but you're talking about ATV and tractor starting batteries).
This is a problem when the CC is fooled into thinking that the battery is discharged when actually something is wrong that's accidentally holding the voltage down. Then the CC responds by opening the throttle and passing as much current as it can - with disastrous results.
I will second the suggestion from BRCM to unhook most of the panels but add my 2 cents: not all of them. If you can hook up just enough panels in a way to offer a small trickle to keep them topped up, long-term that will help them pass through the -25 temps. I have a few of these 1-square-foot panels for jobs like this.
One thing about batteries (bear with me if you may already know this) is that they are more resistant to freezing when they are fully charged. Mine get it worse than yours, and have to take -35F once or twice every winter (not this one it seems, touch wood).