Mine was doing the same thing, checked and checked. Then one day at work, I lift the back of the truck with a fork lift to work on the trailer connector (dont want to bend over) and up by the Transmission, I see fuel leaking out! That quick connect on the fuel return line was leaking. It is up by the bell housing, and normally above the fuel level of the tank, so it did not leak when parked.
When I lifted the tank above the engine, fuel started to pour out! I cut the steel line to remove the entire flex line, and just intalled a rubber fuel hose, 5/16".
Problem solved!