Emergency Power, Second Battery, and a Battery Isolator

I'm a relatively new 2012 Prius owner who was motivated to buy a Prius, in no small part, by the Prius's ability to serve as an UPS and generator in a jam. I've got a 1000w Xantrex Pure Sine Wave inverter and have all the 2ga cables ready to go. Then, an ugly thought occurred to me--- if I run down the 12v auxiliary battery (after some time powering a few appliances during a blackout) and the DC converter is working hard to recharge the battery, I might still blow the 100a DC/DC converter fuse if I have a sudden increase in load (e.g. the compressor in my refrigerator turns on).

After thinking about it for awhile, I think the weak link is the puny 12v battery in the Prius. It doesn't have much capacity (so will be low more often) and doesn't have much power for intermittent high loads (i.e. little "crank power"). A better option might be to use a battery isolator in the line into the existing 12v battery and then have a much bigger battery serve the inverter.

I've created some circuit diagrams of what I'm getting at. If the battery is bigger, it might lessen the chance that of a sudden load on the converter-- both because there will be fewer "low charge" periods and because there's more residual amps in a bigger battery.I've heard the idea of using a battery isolator bantered around on the forum before, but haven't found any specifics.A few questions

1. Is this a viable idea and will it lessen the chances of blowing out my 100A fuse?
2. Having never used a battery isolator before, is this the way to wire it?
3. If I keep the leads to the isolator really short, do I have to fuse it?
4. I'm thinking about the Optima D51 Yellow battery for this application because its specs seem close to the Prius existing battery-- except that it has a lot more cranking power and a lot more amp hours, of course. Is that the right way to go?
5. I'm assuming that a 100A battery isolator is optimal, but what do I know?

 

Okay, moved the discussion over there!