I just bought a deep cycle battery from advance auto. It is dated 2/18 so I assume it has sat on the shelf since then without a charge. Anyways as soon as got home I checked it. It was 12.3v . Then hooked it up to my good Optima battery charger and it said the battery was 50%. Should I return it?

Let it charge overnight then check again.

Ok, I`ll check it again tomorrow.

Just put it on a low amperage, 2-5 amps is all you need.

I charge my two deep cycle group 24 Marine-RV batteries in my bracket car for two or three days in advance of racing it with a 4 amp charger
I let it sit overnight so the battery will cascade down to their steady state voltage I don't rely on peak voltage coming off the charger

I've always heard all new batteries should be charged at a low amperage on an automatic charger overnight. AGM batteries require a special charger, but it sounds like you have that covered.

Throwing a low charged battery in your car and relying on your alternator to charge it is very hard on your alternator's bridge diodes.

Some deep cycle battery designs use a slightly weaker electrolyte in order to get longer life from the plates.

Just like hotter outside temperatures will make the electrolyte expand ( lower Specific Gravity) and the full charge voltage drop slightly, a weaker new fill of electrolyte also makes the full charge voltage less.

The various designs of lead acid batteries can have fully charged voltages ranging from 12.4 up to 13.2 volts.

AGM designs tends lower.
Calcium or Atimony as an alloy in the lead affects voltage.

Charging up a new battery at low amps for more than 24 hours is good practice, let it sit to cool down, then measure both resting voltage and electrolyte SG to know the new baseline numbers to judge when future charging is complete.