Yep, sounds like worth a second look... or three.
First probably useful to pin down which bank really does what. It's not uncommon in that style of
boat to have dual-purpose banks which start one engine and run approx. half the house. IOW, you may or may not really have a dedicated "house" bank. And then sometimes a genset has it's own starter, or is serviced by another bank. Once you can say for sure what's what it'll be easier to help you get to where you want to be.
Doesn't take all that much to start your diesels; maybe a good Group 31 apiece with serious cranking amps. Or two G31s each, if necessary; that wouldn't be unusual. You can look up minimum required cranking amps in your Cummins
documentation. And then also look to see if you have a paralleling switch at the
helm; that'd be useful to know, also.
Genset starter can be a whole separate one-battery bank (G34 is usually large enough) or maybe it can piggy-back on on one of the engine banks. And maybe that's what your port engine/genset bank is about. And then your "forward/center bank is for the starboard engine?
Then if you do have a dedicated house bank -- your "center/left" bank? -- that'd be where deep cycle batteries are better.
Anyway, just some initial comments. It'll take knowing more about your system to home in a bit closer... Might just be you need decent start batteries for your "port/generator" and "forward/right" banks. IOW, could be it's close, just needs a bit of tweaking.
-Chris