If your battery is reading 12.6v after being charged fully, then I'd probably rule the battery out and say your likely issue is a parasitic battery drain.
I'd be inclined to ask the dealer in question, to diagnose and fix the battery drain.
It's very simple to find out if you have got a battery drain in general. And it should be no trouble for a service technician, then they can go through the usual process of elimination to find the drain, and fix it.
Had a similar problem on my wife's Alfa Romeo, thought it was the battery, turned out to be the Bluetooth module draining the battery.