You will wire the thruster to the pos/neg at the bow bank, with its own breaker: So the closest battery bank will supply the peak loads, and the connection between your forward bank and the house bank will just handle equalizing loads between the banks, which are limited by the charge acceptance rates of each bank. Your bow bank gets depleted it will draw more charge from the alternator and house bank until the two banks are in equilibrium and/or fully charged. But the rate of draw in amps is limited by how fast the bow bank can accept a charge current at the voltage supplied.

BTW that 300A max load is probably a stall load, which is a peak you should never achieve under normal operating circumstances unless your battery voltage is too low (less volts requires more amps to deliver the same work). So if you find you're tripping the breaker there's a good chance your battery is weak.