I guess what I am really doing is adding a third battery... primarily for the engine alone.

I have two group 27s now, but I only use one, while anchored someplace... thus giving me effectively only 95AH... I always leave the other battery off line, and in reserve as a start battery.

If I add a start battery, then the house power effectively doubles to 190 AH... while still keeping the starter battery in reserve.

I have a 2GM20F, so sounds like I could used a group 24 easily. Maybe tuck it right down near the engine.