Harry...a typical 100amp group31 deep cycle battery has 1/2 the CCA's as a starting battery. Thus TWO group 31 deep cycles can provide exactly the same CCA's as a group 31 start battery AND provide 200amp hours (100amp hours to 50%) of house bank needs. This is a MUCH better solution than a dual use battery which has thin plates and a short life in deep cycle applications.
BTW...I am assuming you have the same Perkins as I do..and I have the same Balmar setup...but my 8kw gen does my charging typically with a 135amp charger to 1000amphours of 8D AGM's. A very different situation than is being discussed here.
No longer posting. Reach me by PM!