engine won't turn over

One easy way to rule a lot of stuff out: find your large starter relay. It is a tower relay with a big cable from the battery then to the starter. If you jump across the contacts withe screwdriver you are bypassing everything g except the batteries and starter ( and battery cables). If it doesn't crank doing that, it is your starter or cables....since you had batteries checked. If the ignition is on, you can start it that way.

Freightliner can tell you where your relay is. Mine is behind a cover in my electric bay.

Jeff