Low charge to no charge issue pulling hair out

The charger voltages may be confusing. You cannot charge a battery at 12 something. Chargers, just like automotive alternators, MUST charge at least at 14V nominal, depending on temp and whether the batt is lead acid or something else. Even a "float" charger is around 13.5 and that would be 24/7

So, charger disconnected, surface charge gone, a battery should be nominal 12.6. If the charger is "really smart" and can resolve that, then it might show that. I doubt most do.