2001 Pontiac Grand Prix Battery dieing overnight

My car is showing all signs of parasitic draw, but I've tried both the multimeter and the 12v test light method and neither of them are showing results. The battery voltage with the car running and the alternator charging is about 14.5v, the car just turned off it's reading about 12.63v, but within hours, the battery reads as low as 12.3v. I have yet to check the battery voltage in the morning, but I know it's low enough to stop the car from starting, and the battery goes almost completely dead within 2 days. I have checked all interior lights, etc and no obvious culprits. Battery does not discharge itself when unplugged from the car. The battery maintains a steady 12.45-12.53 when unplugged. Also did an ac current/ diode test on the alternator while running and off, car running it has about 30v ac and while off, about 26v ac. Is 26v ac too high for an alternator when the car is off? Could a bad diode be draining the battery when the car is off? I am dreading going to a mechanic shop, I am a diy type of person, I love fixing my own car and I know electrical = money.