Rickilller, congratulations on your purchase and welcome to the forum!
After initially purchasing our i-MiEVs, many of us were obsessed with improving the car's efficiency and thus its range. Over time, we realized that from a practical point of view it simply did not matter as most of the modifications provided a negligible improvement when compared to simply improving our own driving habits. Reality is that very rarely do we need those extra few kilometers and when we do we simply slow down and featherfoot the accelerator to get to that particular destination. In everyday driving range simply does not matter and, in my case, I rarely even fully charge my car and after almost eight years and 100,000 miles of driving the i-MiEV I can still claim that I never saw turtle.
By far the largest energy consumer outside of the car's motive system is the cabin heater, especially now in winter (Northern Hemisphere). The most significant and useful improvement made by our seriously-cold-weather members which you might consider is the addition of a diesel/kerosene/bioethanol heater http://myimiev.com/forum/viewtopic.php?f=14&t=1248
Yes, adding insulation helps of course, both thermally and acoustically, but few of us outside of seriously-cold-weather regions have done that. For myself, if I really need to extend my range in winter I turn off the heater and wear thermal clothing or wrap my legs with a fleece or even a 12v electric blanket (but need to occasionally activate the heater/aircon for window defrosting).
Regarding your concept of a larger 12v battery which you would only charge at home and might have enough storage capacity so as not need the dc-dc during a trip, there are a few different issues -
1. In the overall scheme of things, 12v power consumption is not significant compared with motive power. Other than the fixed draw by the car's electronics, use of the car's headlights or radio or seat heater simply does not matter.
2. The i-MiEV is VERY sensitive to voltage drops in the the 12v system and, we find, most problems with the car stem from this voltage being too low at startup due to older discharged batteries. Avoiding sagging of the 12v battery voltage is very important.
3. After many years of incredible reliability, the Achille's Heel of the i-MiEV is the on-board charger
/dc-dc module (OBC/dc-dc) which suffers from a variety of design flaws in addition to an unfortunate placement location in the car. Replacement of this module from Mitsubishi/PSA is cost-prohibitive as it often exceeds the value of the car. For this reason I personally would not do anything that might mess with that module's operation. Luckily, in the USA Mitsubishi has acknowledged this problem and has extended the warranty for this module to ten years.
If I might suggest, first focus employing the many driving tricks available to you to see how easily you can very significantly extend the range of the car, and only then see if you think making additional improvements will really be needed. You can quantify your experiments by utilizing the Range Remaining dashboard display and comparing it to your fuel consumption gauge. By far the most valuable addition you can make to your car is to obtain a bluetooth genuine OBDLink LX or MX module and one of a number of smartphone apps, with my own favorite being CaniOn for Android OS. This will enable you to quantify not only your driving efficiency but will give you insight into your battery status and many other useful car parameters.
Wish you all the best and do let us know of your progress.