Max torque will be between 3,000 and 4,000 RPM
There are a lot of different cylinder heads and piston combinations for big blocks, so i's hard to say how much power. And "RV cam" could be a lot of things
The short answer to your question about max rpm is "as low as possible", but you should be able to cuise at 2500 RPM or less. Just because you have a reduction unit on your boat doesn't mean you have to double your RPM. Add pitch to your prop until your RPM is as low as possible without losing too much throttle response.