No auto start-stop junk on mine, so only one battery...but being a diesel, it's a BIG battery. And like you, it was a cold snap and my remote start didn't work one morning. Put a charger on it and it would only go to 3/4...so I changed it before it stranded me.
I had the charger on the under hood posts while I changed it (kept the + terminal away from ground), and I didn't even have to reset my radio stations. Fired right up when done.
All you need is a 10mm socket, and an extension to get the nuts off the retainer at the base. I used a magnet on a stick to pull the nuts and retainer (hold down) out. The terminals are a goofy wedge setup, you loosen a vertical nut and it loosens the terminal clamp. You don't have to mess with the 13mm cable nuts.