check the load voltage at the batteries when you put power to the trolling motor.
13.4 (or close to it) volts is what a battery should read (cold).
times 3 batteries 38-40.2 volts.
I have to respectfully disagree, but perhaps you are thinking apples and I am thinking oranges.
13.4 is a typical voltage a battery will read when it is fully charged and the charger is still plugged in and in "float" mode and no load is being applied.
A charged battery at rest, given adequate time to stabilize after being disconnected from a charger or engine alternator should read between 12.5 and 12.9 volts, depending on condition.
Any applied load will drop the voltage from the resting voltage commensurate with the severity of the load.