fuel leveling problems

Same problem here on an 05 379 with a C-15 Cat. My leveling problems started after I had to replace the right side fuel tank. I learned a few things in the process of fixing my problems. First, the truck runs off of both fuel tanks simultaneously. The fuel pump maintains a predetermined pressure on the fuel system. The engine simply takes what it needs to run and returns the remainder to the fuel tanks. In my case, on the back of the head, there is a valve that equalizes the fuel so that an equal amount is returned to each tank. You should have that checked. Also, make sure you haven't gotten some trash sucked up against the pick up tube in your left side fuel tank. The fuel as it returns to the tanks will take the path of least resistance, so you might also check the return lines to make sure nothing is clogging them up. One thing you might do, with the fuel pretty low in the tanks, engine running, take the cap off, shine a flash light in the fuel tank and visibly verify that fuel is indeed being returned to both tanks, you'll be able to see it running into the tank. Another lesson learned is that Peterbilt's are kinda finicky, make sure both of your fuel caps fit correctly and close correctly, if one cap is loose, essentially that becomes an extra vent for that tank, allowing more air to leave and in turn allowing more fuel to be returned to that side. Finally, fuel lines like everything else goes bad. The rubber lining inside the line can start to break away from the line allowing the lines to become clogged.
I think you have identified part of your problem with that one line that is "cool" while the others are warm. Ultimately I replaced my return lines from the engine to the tank as well as the equalizing valve on the back of the head. Problem seems to be solved.