Recommend some basic troubleshooting to see if there is a problem and (with luck) find it right away.

Hook up a voltmeter across a battery and take a reading with the ignition off. You should see about 12.2 to 12.5 volts. Lower voltage would mean the batteries are not fully charged.

Start the truck and read the voltmeter again. If the charging system is working OK you should see 13.5 to 14.5 volts. Lower voltage probably means that the batteries are low on charge or the glow plugs are still on. But the charging system is working.

If everything checks out OK, next look for parasitic loads when the ignition is off. Connect an ammeter in series with one battery, then disconnect the second battery. Read the amperage. This reading is what the truck's electrical loads are with everything off. You will probably have to disconnect the underhood light to get a true reading.

If there is a significant current draw, it is time to start pulling fuses to find what circuit is at fault. Disconnect the alternator during this check to make sure that it is not drawing current when it is off. Once the problem circuit is found, look at each load on that circuit until the criminal is found.

Lou Braun