We will get to negative 40 F at my place and while I got smart and started using smart chargers on all of my batteries, I have in the past left the battery connected in the boat with the battery switch in the off position and been fine. It is an unheated garage where it is at least a bit warmer but have also had the battery freeze and the the battery expand like a pop can left in the freezer thus needed to be replaced. In your shoes and knowing the high cost of batteries, I would do whatever I had to in order to remove them and bring them home to connect the charger in the garage. The worst batteries are power sports and lawn mower batteries...They need to be kept on a charger or you will need to buy a new one. My experience with solar is that I have a 24 volt boat lift that has a solar panel system and charging/maintaining system that keeps the deep cycle batteries charged all summer even in low sun MN. In the winter, they go into a heated garage with the smart chargers on them. That solar/charging system is terribly expensive.
