go with the dual batteries and the HO alt, then...
two optima yellow tops, should run you around $160 apiece for them (depends on where you go, I bought mine at a local auto parts place).
You'll shouldn't need an isolator between them, as they're identical batteries, but you'll need 1/0 or 2awg cable to interconnect them, and properly-sized fuses. Wire them up in parallel to get 12V but with a higher Ah rating.