Dual fuel tanks, returning fuel to wrong tank?

[hartbraker]

So, when my selector is on my driver's side tank, the truck runs just fine... but it seems as though the return line takes the fuel back to the passenger tank.

At first I thought my eyes were playing tricks on me. But with the passenger tank empty, I tried to confirm my thoughts, and sure enough when the driver's tank was empty, I switched over to the previously empty passenger tank only to find 3/4 of a tank of gas.... I'm thoroughly confused.

Do I have a plugged return line, a faulty selector valve, or what?

See signature for truck specs. I put up another thread in the engine/drivetrain section but it wasn't getting much traffic. Thanks in advance guys/gals!