2000 Freightliner FLC120 fuel question.

Good advice already suggested also try filling up one tank at a time and see if they take more or less the same gallons on each side...

 

Might be the GT return flow splitter. It’s located on top of Drivers Tank. Has fuel lines running to it. This explains how top draw tanks work, the gt flow valve regulates the return fuel. Also the Fuel guage is usually on Drivers tank. Need to check vent hose on other tank first. Might just be plugged. You can simply try to blow through it, with or without the cap off. Caps themselves have a vent. Usually don’t go bad though. Taking the cap off, letting it sit a while, if the vents the problem, Tanks should even out. Might take 1/2 hour, depending on fuel level. Not sure.