When you say 12.5 volts indicates a pretty dead battery, what should my voltage read if my MC is not running and not connected to a trickle charger. The voltage meter was installed by the previous owner and I don't have the knowledge to figure out where it is connected.
It depends on: what type of battery (Gel or AGM), battery temperature, time since last charge or since last engine running.
At a nominal battery temp (not real hot or real cold) then most good condition fully charged AGM 12v batteries will show in the 12.7v-12.85v range.
At a nominal battery temp (not real hot or real cold) then most good condition fully charged Gel 12v batteries will show in the 12.8-12.85+v range.
The above voltages are for a direct battery post voltmeter connection. If the voltmeter connection is made to wiring in the vehicle harness, or post ignition switch, then it could be a tenth or more lower.
Static battery voltage is just a quick status, to really know battery condition it should have a proper load test.