Will give you a 5% voltage drop. Since you do not say the full distance to the main bank I don't have the full answer. As was said by others, 300 amps is the motor stall current so you could probably get by with the next size down. Do you have a large alternator? You will need to consider the alternator wire size as well. Don't forget fuses or circuit breakers for both battery banks.

I got this figure from Bestboatwire.com