To winterize:
• Charge the battery fully.
• Disconnect every load and cable.
• Store it with nothing attached.
LiFePO4 loses only about 1–3% of its charge per month, so it holds charge well over the off-season. Millertech’s “How to Winterize Your Boat’s Millertech Batteries” video walks through it.
How to Winterize Your Boat's MillerTech Batteries

• Charge the battery fully.
• Disconnect every load and cable, or leave a solar panel connected with the controller on a lithium setting (see "Can I Leave My Solar Connected While My RV Is in Storage?").
• Store it with nothing else attached.

LiFePO4 loses only about 1% to 3% of its charge per month, so it holds its charge well through the off-season. Millertech's winterizing video walks through it.

Yes. Millertech batteries perform “extremely well in cold conditions during discharge.” Sport Series batteries are rated to run from -4°F to 140°F. The limit is charging in the cold, not fishing in it (see question 25). Greg Hackney ran his while breaking ice: “nothing but raw power, the same as if it was 90° every day.”

For long-term storage, fully charge the battery, remove all loads or connections, and store it with nothing attached. This preparation helps maximize the battery’s lifespan during extended storage periods, such as over the winter.

During your active season, yes. Keep the battery on the charger whenever you’re not using it. The BMS and the charger’s own cut-off together mean it can’t be overcharged. For storage of a month or more, follow the instructions on this video:
How to Winterize Your Boat's MillerTech Batteries