the in cab guages usually arent all that accurate, im talking about a voltmeter, like the actually test tool, not the stock guage...
have you checked the fuses? how are the ground wires?
disconnecting the battery and if it dies usually will point to charging system, do you have an internal voltage regulator or external?
get a multimeter and check its output, a properly charging system should produce around 14 volts of output
