Peterbilt 387. Dead batterys again.

DONT just put a switch on it and ignore the underlying electrical problems. It will come back to bite you. With 4 batteries it can and will create havoc quickly.
This is how I know.
Luckily, it was a day AFTER I got home and not when I was sleeping in the truck... started between the battery box and the harness out to the alternator/starter.
GET YOURSELF A SMOKE DETECTOR. REPLACE IT EVERY YEAR.

 

I know you said replaced the cables but most folks only replace the positive cables. I would take a look a look at your negative jumpers that connect the batteries.

Wow! Just realized how old this post is. Pretty sure OP has moved on

 

Im having a similar problem in freightliner my battry die within 5 6 hrs even with the kill switch off but it starts it up without boosting if i wait and let the voltage get to 12v not sure where to start looking

 

Check the negative cables connecting the batteries.