Given that the typical Lithium battery can't be recharged below 32F, I did some digging around and found
thisHas adhesive so it sticks to whatever you put it on; On/Off switch you turn/switch on when it nears freezing, and it has a thermostat that, when powered up, comes on at 45F, and shuts off at 68F.
It looks as if a couple of pads, wrapped around the sides of the batteries, would do the trick...
These were initially designed to adhere to fresh water tanks of 50 gallons, with more pads for larger tanks...
These days, however, I'm not sure if mine rig, or any others out there, already have a system where pipes and tanks were kept warm enough using the RV's power so you wouldn't have to be concerned about pipes, etc. freezing...
Thoughts?