I would not be using a marine deep cycle for the starting batteries Paul. You will get a better start from an Automotive cranking battery. I assume your start batteries are isolated from the house circuit, so why would you want deep cycle for start? (They will have less start and take longer to recharge with the alternator.)
What does your 5.9 call for in the way of CCA? Probably around 800CCA.
My 8.3 has twin GP 31 950CCA start batteries. When one failed in summer of 2011 on the road, I tried to run with only one battery for the remainder of the trip, as I had a good 8D battery in the shop at home. I found the dated single battery would barely start the 8.3 and caused, what I think, the ECU to reset each start, resulting in slow start, smoke, and rough idle for the first moment. http://www.airforums.com/forums/f313...ml#post1009548
My point is, I think you would be better served with start batteries rather than deep cycle, for the chassis circuit.
I would still go with two batteries which would allow some emergercy use of lights and etc, and still have capacity to start the unit.