Fuel problem C15 MBN

Your truck draws from the left tank and the fuel returns to the right tank. Does your truck shut down out of fuel with the left tank empty and the right tank full?
Look at your tank vents and fuel return lines. Either could be plugged or collapsed to the point of preventing flow.
Good luck.

 

Some have crossover valves that fail after a while. I'd just replace them with a standard T fitting and be done with it.

 

What truck?
Pete's draw and return from both tanks. There is like a Reed valve on the return that basically senses pressure and returns fuel to the appropriate tank. That can get crap in it and stick one way or another. Or most likely your tank vent is plugged and pressuring a tank so fuel doesn't return to it.
Then so magic voodoo happens when you park and the tanks will even out, or if parked on a slope it will go to the low side. Surface tension or siphon or something.

Pull your tank vents and see if you can blow air through them

 

You have locking fuel caps? If you do take a look at each cap and see if one is missing. If one is, find a magnet and fish it out of your tank.