Do not hook up 12v too a 24v series battery.This is detrimental on the batteries.When hook up to a single 12v in 24v the single battery is only being use.
The other battery doesn't push amperage into the battery being used for 12v.So now you have two different amperage's in the batteries.Then you turm on the trolling motor,
Increasing the amperage draw from the 12v hooked up battery.More draw the hotter the battery gets causing damage.
In a multiple battery hook, Your batteries are only as strong as the weakest one.
You must use a 24 to 12v inverter.