Tesla doesn't display the 12 V battery voltage, unlike several other EVs.
I would recommend adding a
12 V Auxiliary plug with a voltmeter, or use
Scan My Tesla to check the battery voltage.
From what I noticed, I general the battery voltage is around 12-14 V
and should be around 14-16 V when the 12 V battery get charged.
When the car is asleep, and there is only the phantom drain when park,
it would be simple to have a voltmeter connected directly to the 12 V battery in the frunk.
After locking the car for few hours, if the battery voltage was going well below 12 V,
this would be a good indication that the battery didn't get charged.