I would not hook different types of batteries in parallel. They will likely perform poorly, and if one has a bad cell, it will draw down the others. That's one of the advantages of running 2 6v's in series.
Perhaps you could set up a heavy duty switch, so you could switch between your two different banks of batteries?
Otherwise, if you need more capacity than the 2 Trojan T105's, you could go with 2 Trojan L16HC's. They are taller than the T105, but are rated at 420 amp hours each...
BTW, I see this was your first post. Welcome. I'm sure lots of friendly people who know a lot more than I do will also chime in to offer help.
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