Peterbilt 387. Dead batterys again.

Gotta ask the obvious, do you have an inverter that's staying on?

 

do you have DEF? there is a heater in that.

do you have a fridge?

the computer systems do take a very small amount of battery power, but that should not be enough to cause the batteries to go dead.

the only other thing i might suggest is some sort of "battery tender" to keep a trickle charge on the batteries.

 

How old are the batteries

 

i wonder too how good and adjusted is the alternator belt, and the alternator output for both amp and volts. it the alternator is not producing enough, it will not keep the battery at a full charge.

 

Easiest solution would probably be install a Killswitch at the battery box

 

That's got to be a fairly big draw. If you have a temp gun start taking readings in you fuse box and any where you have a relay after the truck has set a while.