parasitic battery drain

I think I read that under 0.1A parasitic draw is acceptable, for a vehicle that sees weekly use. A typical car battery is about 100 A-h, so after a week it would be down about 17%.

The 0.3A would be too much. That may be the load of a small lamp or a relay coil or two. Glove box, or trunk lights come to mind because when closed they cannot be seen as on. Another place to look is the fan relay, they often run with the ignition off. If the fan is inoperative, the relay may draw current with a defective temperature sensor.

Modern electronics should get the job done at much less, perhaps 20 mA or less. Owners manuals often call out a certain fuse to pull for extended storage.