what should I do to fix this
Start probing the electrical components to find what is draining your battery. Pretty hard to tell you what to exactly look for.
You have something draining the batteries or you have a dead cell in one or both batteries. A battery with a bad cell will crank until cold weather demands more cranking power. Have your batteries load tested. If one is bad replace them both in a diesel with no lower than 1000 cca batteries.
If you have a draw look for a light staying on. Glove box,under hood,dome light,door switch lights.
Replace the alternator
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