The graph for OC voltage vs residual capacity shows a bandwidth. I see Full can be from 12.8 to 13.3.
I suppose you have to discover exactly what yours is if you need to know at all. If you are always either camping or on a Float charge you would never see that.
No ammeter in the TC, so to discover if full = zero amps would be a special test set-up. Need to fine tune the float voltage to the self discharge per temperature but that would require an ammeter too.
So far this one simple AGM is looking like I need two of them plus another Trimetric for the TC, a new battery charger with adjustable amps,------