I was very concerned about this. So I googled it. Here's what Renogy says about my batteries:
【Low Temperature Protection Function】 The lithium battery has low temperature charging and discharging protection function. When the temperature is below 0℃, it will stop charging. When the temperature is below -20℃, it will stop discharging.
Every company has different requirements, so check what your batteries need. I'm in PA where it rarely gets that cold. But for when it does, I bought this:
https://www.amazon.com/gp/product/B083FJ7GKR/ref=ppx_yo_dt_b_asin_title_o09_s00?ie=UTF8&psc=1
I'll set it to 38 degrees and have it turn on a heater that's plugged into an extension chord from the house or to shore power. I'm leaning toward the extension chord. Why plug it into the van then plug the thermostat into the outlet, when I can just plug the thermostat into the extension chord?