... the voltage reading was at 12 volts. Is that just because it is the "nature of the beast"? I always thought a fully charged battery was supposed to be closer to 12.5 volts...
An AGM battery is a normal Flooded Cell, Lead Acid battery with Fibergalss Mats placed between the plates of the battery.
It will behave just like a normal battery with the added advantage that if turned on its side, or upside down, the acid will not flow out from between the plates.
It should show the same voltage behavior as a normal battery.
At a full charge at room temperature you should see 12.6 volts.
You should also have been seeing ~14 volts or more while charging, and the voltage slowly returning to 12.6v over a few hour period after charging.
Are you certain it was actually being charged? Was the 14v+ present?