2000 kodiak not draining both tanks

[towtruckkodiak]

So I drive a 2000 kodiak rollback tow truck. It has the cat (3126..?) Motor. Ive got dual step tanks and for some reason only the driver side tank drains. There is a fuel sender in the driver tank, the passenger only has a vent on top and a petcock feeding into a line at the bottom that runs to the bottom of the other tank under the trans. It runs great drains the driver tank fine but the passengers is full never empties. We drained the tanks replaced the line and checked the petcocks, checked both vent lines which appear fine. The driver tank has pressure when you open it, passengers does not. We have no idea what's wrong and it's doubled our fuel stops getting about 5-7 mpg. Any help would be appreciated