I'm going to speculate that you have a small battery drain that doesn't effect you as much through the warmer months due to the glow plugs not being needed.
Would end up shortening the life of the batteries though by constant depleting and recharging.

Are you running sealed batteries? Could they maybe use a little water?

Have the alternator load tested also. 14 volts at 40 amps and 14 volts at 100 amps looks the same on a volt meter.

 

How do you have the alternator load tested? Autozone? Does the alternator have to be removed? Has nothing to do with batteries being bad b/c it's been 3 sets in a row. After the truck has been started it will start up fine that day and then need to be jumped the next morning again.

 

Sounds like you have a drain on the system-- first place to check is any aftermarket items installed--

You can test the drain on the electrical system, but first, I suggest you disconnect both battery negative cables-- let it sit and for a day or more if you can-- install the cables and see if it starts. NOTE--You will see a spark when reconnecting the cables, as that is the battery recharging the truck computers

Do you have a hand volt/amp meter-??

 

I leave the truck plugged into a trailer and the trailer has 2 dead batteries. Could these batteries draw from my truck batteries? Not sure if this is the problem though, as this truck was a safety highway truck b4 I got it and had a lot of electronics. It's almost impossible to find a slow draw if there is an electrical short somewhere. Had a mechanic a few years ago check for a short and he couldn't find anything.

yes I do have a volt meter.
how does unplugging the negative batteries test the drain?

 

The 12 volt cable in the 7 pin connector, is HOT all the time. Anything connected to the truck, can pull power. If you have a battery in the trailer and the connector is attached to the truck, it will try to equalize the volts, and reduce voltage from the higher volt battery. Of course a bad battery in the trailer can deplete the truck batteries over night.

 

With the truck off, unhook the negative from the batteries and connect a multimeter between the battery and the negative cable, set he meter to read Amp draw and see what the reading is. Once you have that reading, start to disconnect things starting with the trailer, disconnect circuits in the truck by pulling fuses one at a time and checking the system draw until you find what is drawing current. That will give you a good starting point to finding the issue.

 

This test will not work unless you disconnect the other battery after the meter is connected-! The batteries are connected together to make one big battery. Also make sure the trailer connector is disconnected.

 

Thats why I use the plural "batteries", disconnect them both and test on one. Also check the voltage of each battery after disconnecting and letting it sit overnight, any sizeable difference will make the stronger battery bleed off to try and bring the weak one up to its voltage.