Have both batteries load tested?
Parasitic draw? What vehicle built these days doesn't have them? So many computers and stuff in vehicles now....
We have a 2019 Highlander. Was sitting in a parking lot waiting on my wife while she was in a fabric store. Ignition in accessory position, radio on nothing else. The master alarm suddenly goes off and the dash display pops the following message " shutting Ignition off to conserve battery". Radio only????? After 20 minutes????
Good thing I can stream Sirius XM on my phone. Geeez.......