1988 Ford Motorhome 1987 E-350 electric fuel pump problem

The electric fuel pump acts like it is not receiving power however 12 volts is being carried to the pump. What would burn up a pump with in 5 t0 10 minutes this is the third pump I have installed. following items have been check and or replaced, The inertia switch, the fuel relay the oil pressure switch, fuses on both sides. I am absolutely lost now. I have checked all wires.