what should I do to fix this

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