Truck runs real bad and terrible fuel milage with a 4.3 litre engine

Good chance that the fuel pressure regulator is leaking gas and needs to be replaced. The fuel pressure regulator is located at the base of the throttle body and when it's starts to leak when the engine is running, it starts to flood the engine causing it to run bad and rich in fuel. The other common thing that would cause this is a faulty EGR valve, due to carbon build up and will need to be cleaned or replaced.
Do a fuel drop down test, test pressure line with gauge attached...leave for a bit...does fuel pressure stay close to where it was or drop off fast... if it drops off test the fuel pressure regulator by pinching off the vacum line...while running...pressure should increase...if it doesn't then replace pressure regulator.
Good luck and keep me posted.

Could be any number of things but try changing fuel and air filters if this does not help I would have the injectors checked