Uneven fuel level in tanks.

[DVT873]

I don't know about Freightliner but on most NAV units there is not an equalizer line across the bottom of the tanks. Instead there is an electric fuel pump that moves fuel from one tank to the other. Draw tank is usually but not always on the pass. side. Body computer compares the levels and moves fuel as needed. Engine has to be running for transfer to happen. There is an inlet screen in the pump that occasionally plugs up. Pretty reliable system now. Very very early they tried using a 5 wire pump that read the sending units directly. Lots of problems. That was gone by mid 2000's.
Before that it was dual draw dual return. Those had some problems. All the lines had to be on the same level. No kinks or dips. You would get to much fuel returning to one tank.
Be best to figure out what you have.
If there is a sending unit in each tank there is likely a pump somewhere. The NAV dual draw dual return only had one sender.