Cruisee,
If there is any way you can fit a pair of GC2s in that house battery space, do that.
If you aren't sure, find some boxes and packing tape and make some. We did a lot of boat electrics before the depression, and I used to keep a pair of cardboard (much lighter than the real thing) in the coach just to used for show to owners.
The other thing to be aware of is that 12V real deep cycle batteries are both rare and expensive. Marine batteries are not. CCA and CA can have little to do with actual capacity. If there is no Amp-Hour number available, look at the Reserve Capacity. That is the number of minutes you can leave the headlights on and still hole to start. This is a real number.
Matt