The gas line can't be completely dry, otherwise it wouldn't run at all. You say it will run as long as you are pressimn on the gas pedal. I think Tester (above) is probably right. That's where to check first anyway, imo. If that brings no joy, a fuel pressure test is probably next.
Edit: If you feel lucky, it cold be the fuel pressure regulator too. Not sure if your truck uses one though. But if it does, and it is part of a fuel return system which returns unneeded fuel rail fuel back to the tank, ask your mechanic to remove the vacuum hose connected to it (which goes to the intake manifold) and check for any fuel inside. There shouldn't be any.