Had the same problem with a boat
I bought a while back. My brother who was a diesel
mech in the Navy
fixed it by replacing all the Rubber Fuel
lines from the Tank to the filter and high presser pump. With new lines and filter element it was literally a $30 fix. He say's with fuel problems start at the source and work
your way to the more expensive fix's most people don't know fuel lines rot
from the inside out and won't change them until they leak on my boat
they were both cracked on the outside and rotten on the inside even though the were not leaking. If the pickup in your tank is on the top don't forget check to make sure the tube that goes to the bottom of the tank is still attached and not cracked at the top (Had this happen too!) even a small crack will make it stop drawing from the tank.