mpg decreased a lot after "stay at home"

Most would agree on getting a float charger. I don't drive much since quarantine so I put my battery on mine once a week and let it fully charge. I put the red clamp on the positive battery terminal and the negative clamp on a chassis bolt.

https://www.amazon.com/gp/aw/d/B0051D3MP6/ref=ppx_yo_mob_b_inactive_ship_o0_img?ie=UTF8&psc=1

 

^^^Even easier to attach ring connectors to the battery terminals, and carefully snake the plug wire (securing with wire ties) out of the grill. I haven't done it to my 2 prii yet, but will in a few days. I've done this on all my cars for years (especially beneficial in TX heat, which kills batteries).

Having the plug accessible makes it simple to plug in the trickle/float charger. I've used BatteryTenderPlus 1.25 amp for years. It comes with the wires I've described above.

 

first you need to determine if the 11.6v on the dealer printout is under a load test (presumably it was) then you need to find out what the acceptable voltage range for a healthy battery is under that load. hopefully, the dealer will know these things.

a static read of the voltage after the car has been off a few hours would be somewhat helpful as well.

unfortunately, most dealers don't know much about 12v batteries. they just put the tester on them and go with the results.

all that being said, i don't think the 12v is the cause of your drastic loss of mpg's, although it may or may not be a contributor