Batteries in winter boon docking

[AbbaZabba]

I've been stretching the use of the camper into winter, mostly boon docking but having a genie to charge the batteries. Last week it was down into the single digits and the batteries take a small charge but they discharge pretty fast. Main issue is the last couple of hours at night the batteries don't have enough juice for the heater.

Been searching online and YouTube for ideas on how to improve the battery protection in the cold. Camper has the batteries externally mounted on the tongue, I do have a front storage that I could move the batteries into, but that would consume some useful storage space. Also considered building a box on the tongue that I can run heating mat into just to insulate. Also looked at lithium batteries, but the cost......notable.

Has anyone seen upgrades or mods to resolve? Most of the search results are about winterizing and removing, Im trying to go camping year round.