No voltage measured across "12V" battery after recharging

That Diehard charger/maintainer only puts out 2-4 amps, which would require more than 24 hrs of constant charging for a near dead battery. The reason it takes so long to recharge is the battery terminals are still attached to the car, so the charger needs to overcome the cars maintenance current draw before putting anything into the battery. That usually takes about 0.5A away from the charging process.
For example, sometimes I forget to hookup my fishing/camping truck for a few months and the battery goes down to 4VDC. It takes at least 12 hrs @ 6A to get the battery back to a point I can start the truck. I do have a 12A jump/boost starter, but that's really hard on a battery. Like a good BBQ, low and slow is a lot easier on a battery.
You may want to run some wires up into the trunk area, so you can easily plug-in for maintenance charging, if you leave the car unattended for long stretches - especially if your going to plug back in that Metromile scanner.