You may have a voltage regulator problem. The Voltage Regulator is the device that sits in between the alternator and the battery. Its purpose is to allow current from the alternator to reach the battery when the battery needs it. It opens when the battery is full to prevent overcharging and closes when the battery is drained to allow it to charge. Try hooking up your volt meter to the battery and turning on the lights and fan with the truck running. Then watch to see if the meter increases to around 15 volts and then drops to around 13 volts. If you are not getting this fluctuation, you may need to replace your voltage regulator.