Do not connect locators to trolling motor batteries. As far as disconnection trolling motor, I have seen it reccomended, but I have never done it.
Depending on the trolling motor, the internal circuit boards could be damaged if left plugged in while charging. Your depth finder shouldn't be connected to your trolling motor battery.
I have noticed that my deep cycle battery charges better when unhooked from The trolling motor. It continues to draw a charge when hooked up even after a full day on the charger if I don't unhook it