You must isolate the forward bank from the house bank by some type of battery combiner. Without one your house and forward banks are really one big battery bank. When the voltage in one bank drops below the set point of the combiner it will isolate the two banks from each other and when the voltage rises during charging it will combine the two. By installing a battery combiner you avoid that your house electrical use will drain the forward bank, and the thruster or windlass the house bank. The maximum current flow in the cables between the house and forward banks will be the maximum charge current that your alternator or battery charger can produce. Size these cables to this current flow for a 30ft length. Don't use a diode type isolator, they cause a voltage drop.