I recently replaced the alternator on the truck.

Do you have external voltage regulator. That could be the problem? I'd want to use digital multimeter to double check dash gage. With key and everything off, take reading across battery posts. Note the reading. Ensure all electricals turned off. Connect voltmeter to battery. Attach tachometer and start engine. Operate at 1500 rpm with no electrical load. Voltmeter reading should increase, but should be no more than 2.5 volts greater than battery voltage. Reading should be taken when voltmeter stabilizes. With engine operating, turn heater or ac blower to high position. Turn on headlamps to high beam. In crease engine speed to 2000 rpm. Voltmeter should indicate at least.5 volt more than battery voltage. If voltmeter reads 2.5 volts or greater than battery voltage, stop engine, and connect jumper wire between voltage regulator base and alternator frame. Repeat charging system test. If voltage is less than 2.5 volts greater than battery voltage, check ground connections on alternator and regulator. Repeat test. If overvoltage condition still exists, disconnect regulator wiring plug and repeat charging system test. If condition is corrected, replace regulator and repeat test.