If every thing is charging "normal" and amp gauge is showing discharge
then someone has bypassed the amp gauge.
Look for a wire that runs from the battery post on the alt to the + side of the battery......

If the alt guage is bypassed and alt guage wires are left hooked to the old to the rest of
the harness it will always show discharge since the power for every thing
runs off the power feed from the alt side of the alt.

For my 2c on volt vs amp gauges, take a amp guage any day, however
both used together are better than any one type......
volts just are not as important as amps in any system, you can have
all the volts in the world, without enough amps there useless......
no amps, no volts