Seaking, while wet cells and AGM can SOMEtimes be mixed, they usually have different feeding requirements and that alone is a bad sign.
I don't think it can be overemphasized: The whole charging
system, types, capacities and voltages, must be matched for optimum performance. Not just what the regulator
is supposed to be set for, but also confirming the output with a meter.
I'm not sure about having separate starting batteries for the engine and genset. If they are physically close in location--one battery
ought to be enough for both, since starting either one would in theory give you alternator
power to start up the rest. And a 105AH starting battery sounds like a Group31 in a US size, which is enough to start a honking HUGE engine.
It sounds like the PO either set things up in a way that allowed stray discharges, or didn't match up voltages and battery types. With that many batteries, even at wholesale prices (you lucky SOB! < g >) it pays to go over the whole system.
Then again, maybe the whole system Is OK, and he/you have just been deep cycling the batteries too far. When you push them past 50% capacity, the number of cycles you can get from them generally plummets.