Look at GYLL, I have a 100AH running 3 12” units and 1 of the 3 is a Garmin 1222. The Garmin has a voltage readout on the screen and it’s yet to drop below 12.4V and I generally fish 7-9 hours per trip. Look up David @ Shut Up and Fish in Lewisville, he’s a distributor.
I have the same exact setup with a Nocco charger set on Lithium. It works great. Call David at Shut Up and Fish and he will hook you up.