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You are describing a case of a bad alternator.
Test the alternator and see if the diode is bad or if the voltage regulator is bad. When the alternators go bad, it will provide just barely enough voltage for the truck to run when started, but as soon as it gets a draw on it from another source, the diode goes in reverse and pulls voltage instead of sending voltage, killing off the power to the nengine and it dies.
I've experienced that issue more than a dozen times in my career.
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