Fuel tank showing empty but it’s more than half full.

Problem is somewhere in the crossover line

 

Back in about 1999 I was assigned a brand new Volvo 660. I fueled at the yard and drive to Knoxville from Nashville. It was nighttime. Got to our drop yard in Knoxville and went to bed. Woke up to the sound of the engine dying and a small river of fuel running from my tractor.

Turns out the dealer hadn’t opened the above mentioned crossover line, causing fuel to escape out the vent in the cap. Entire side of the trailer was covered in diesel. When my company checked other newly arrived units, they found several others with the same issue.

 

They haven't used crossover lines in years.
It's more than likely a clogged vent or a clogged or broken pickup tube.
If this is a new problem that's where I would start. I can only estimate that the OP has had this truck for a while and it's a new problem.
If he does happen to have shutoff valves for each tank on both the feed and return line, check to make sure they are both open.

 

Let me clarify that. By crossover line I mean the line that used to go under the tank from each side. There are lines on the topside that go to a splitter valve. That sometimes can be the problem too.
Also, if you have shutoff valves make sure they are both open.

 

Yeah a crossover