I think we're going about this from the wrong end, since you've replaced the relay, and you evuntually get a response from the pump even for a short time leads me to believe that the problem is with the pump and not the wiring, so let's test that first. You'll need someone to help turn the key. If you go under the coach to the fuel tank area you'll find the harness at the drivers side frame rail near the front of the tank. there will be 3 or 4 wires in it. if you probe these while cranking the engine one will have 12v + to it. If you have power you should here the pump run. to verify you can run a jumper wire to the wire that was hot during cranking, to a positive source, the battery . If it's not the pump the engine will run, but with the problem your describing i think you'll find the pump is your problem.
Thank You, Jeff
As I stated earlier the fuel pump runs while the starter is engaged. After the key switch is released to the run position after startup the engine dies after 35 seconds when the carburetor bowl runs out of gas. I have found out there is a oil pressure switch which serves as the power source to the and fuel pump relay and the fuel pump to continue running the engine.
I do not agree with the faulty pump reply. Thank You for getting back to me.