Garage checked the charging system after a charge the battery was getting 12.5

So where do I start my daughter bought a new car so we're going to sell this one. So I drove about 70 miles back to the city To put it up for sale I parked the car overnight when to take the car to vacuum and clean up the windows and noticed on the drive the gauges were not working. So I turned the car off started cleaning and when I got done the car would not start, jumped it off barely made it back to the garage checked the charging system after a charge the battery was getting 12.5 with the car off started FINE but still no gauges checked the voltage at the batteries started off at 12.1 and slowly started decreasing when it got down to 6.75 volts the car died. So I changed the alternator checked all of the grounds mega fuse I have continuity From the battery to the alternator change the battery and still no gauges same problem after running three or 4 minutes if that the volts drop. Checked Volts at the back of the alternator same issue volts steady drop off change the alternator again same issue and still no gauges, the dashboard lights up but no pre ignition warning lights illuminate nothing the gauges don't sweep I'm going crazy please help