That truck is incapable of having an alternator smaller than 95 amps and most likely it is a 140-amp large frame model.
I see posts above that suggest checking amperage between truck and towed vehicle.
140 amps?
Connect trailer to truck. Then measure truck battery voltage. Low? You bet. Check trailer battery. It's either static or losing voltage very slowly.
The alternator isn't charging when the towed vehicle is plugged in. It isn't a drain; it is an intermittant fault. It's time to take it to a *specialist* or to the Ford dealership. The harness under the hood is miswired. The voltage regulator should be in the computer.
Say these words to the service writer "Alternator stops charging when trailer is connected" Simulating the problem can itself be difficult. It may be that an appreance of voltage at a misconnected wire up front shuts down the "C" wire function that runs between the alternator and vehicle ECM. That among a hundred other things will shut down alternator charging. The tech needs to do a point-by-point process of elimination. If the charging connections were done by the previous owner. A good tech can spot it in ten seconds. If the mistake was done by Ford, smile, it'll be their nickle, start to finish.
Use the volts selection on your new clamp-meter.
