12V battery died again

Months ago I posted about my Prime 12V battery dying, and the knowledgeable people here explained the Prime battery facts of life to me. What I should have done instead of using the accessory position, was to turn the car "fully on" ready to drive in order that the big battery keeps little 12V happy.

So today while wife ran some errand I waited in the car. I turn the car "fully on" and verified that in fact car would actually move in F or R gear. I then pushed the park button and started reading a book. About 30 minutes later the radio went off and the dash indicator said "12V battery low, see owners manual." I turned off car and when wife returned much later, luckily the car started and we drove home.

Obviously having car ready to drive, but not actually driving, does not "connect" big battery to keep little battery charged up.

Am I missing something as in a work around would be to put on parking brake, keep foot brake pedal depressed and car in F position so big battery actually keeps 12V alive?

 

Thank you VelvetFoot for the information. A lot of good stuff, but not sure how much is applicable to my "older" Prime.
Yes, my Prime is a 2018 so what generation category should I be looking in? I saw plug in and troubleshooting and went here thinking where my Prime is.