What is your gap between drives? It will go to sleep in 14 days regardless of the voltage. As far as a higher powered battery, I'm not sure one would fit.
Another thing to realize is that the 12V battery is used quite a bit differently in a hybrid. It doesn't start the engine, and it isn't charged by an alternator. It's manly used when the HV battery cannot power the electronics. The HV battery can directly power the entire car through it's DC converter when the car's running. Some of that power is used to charge the 12V battery which acts as the electronics power source when the vehicle is not running (and the converter is inactive).
We drive ours most days, maybe 4 days off at the worst. We're in North Texas, and have had two cold snaps where it didn't get above freezing, but those only lasted 3-4 days each. So far we're clean for sleeps.
