We would put "sight glasses" on inlet and outlet fuel lines, and run engine at full rpms, then bypass a little fuel, to simulate roughly 20 GPH, into a 5 gallon bucket = this additional fuel flow helps "find" problems and gives a better "real life example" to monitor those sight glasses.

We would also have a fuel pressure gauge hooked up(near hand priming pump) and fuel inlet restriction gauge (on inlet going to transfer pump) .

We would verify that the fuel system held fuel pressure , AFTER shutdown, to confirm return line check valve was acceptable.

If engine, didn't already have a CAT fuel hand priming pump, we would install one (those have 2- one way check valves, inside and they help to "hold fuel" in the engine's fuel system, after shutdown).

Now, if all that checks out good ( = not sucking any air), and with your type of complaint (looses prime over 2-3 days) then we would probably change the fuel transfer pump = its shaft lip seal "might be" letting air seeps into the truck's fuel supply system, over time, very slowly; thus allowing the fuel in the truck's fuel lines to slowly empty backwards into the fuel tanks.{note: it gets "air", thru the weep hole, in the transfer pump housing}

After a few days, of non-running, and you begin cranking the engine, the transfer pump only has "air" going to it, thus it can't create fuel pressure, thus a "No Start" condition