Tank problem

Have you checked the cross over tube from tank to tank?

 

Some IHC's used a electric fuel pump to pump fuel from the right tank to the left. System uses sending units in each tank and computer decides when to turn it on and off. Ours had a bad fuel pump.
Look where your fuel lines go. Others are duel draw and dual return.

 

2 things, it's either nor returning to 1 tank, or not sucking from one tank. You can usually see return fuel with the engine running, or remove the return from the tank and run into a bucket. See which side returns faster.

You can do the same with the suction side.

Not sure about an electric pump, but that truck is not familiar to me. Usually you have 2 suction lines from tank to a tee. And 2 return lines. Normally your return will have a splitter where both lines from the tank attach. The splitter controls equal fuel diversion, and if mounted leaning, will return more to the lower leaning side.

 

If you have two lines to each tank that would be duel draw duel return. No pump.

 

Almost every time I have ever had this issue the tank that is full is not sucking out due to trash or who knows what in it. I have found little tops to oil caps, panty hose, candy wrappers.. We had one in the shop one day and my dad (R.I.P.) took the line off the tank that was not sucking fuel out and put his mouth on the line and tried to suck fuel out. Well it was clogged so nothing came out. He blew in it and tried again and got a mouth full of diesel. I laughed at him and asked what did he expect. : )