Measure battery voltage at the battery, and load test the battery . (any auto parts store should have a load tester--and do it for no charge.)
What the chart plotter is seeing is the. voltage it is receiving. As mentioned there is often a voltage drop due to corrosion. Remember that the ground system is just as important as the positive system. Every year, go over all of the connections, check for corrosion-put on non corrosive dielectric grease and tighten up the fittings.
Also often C Dory did not put large enough wire running from the batteries to the console. When I buy another C Dory, I double the size of the wiring--I also put in a large fuse block--and then run each item off its own circuit.
If the battery voltage is 11.1 or 11.3, at the battery it is shot. Generally you should not discharge a flooded lead acid battery to less than 12.2 volts, resting steady state. (nothing running off the battery for at least 30 minutes).
Unless you have one of the newest "smart chargers", leaving a trickle charger is probably not the best practice. Electrolyte needs to be checked regularly in any flooded lead acid battery.
If you take care of batteries they can last a long time. I got over eight years out of the starting batteries on my last large RV (Starting an 8.3 Liter Cummins Diesel). However most small boat batteries don't last more than 3 to 4 years. Although many don't bring batteries up to full charge after using the boat--relying own the engine to charge the batteries, this is not necessarily best practice. The battery needs to be topped off after your use the boat.