I doubt that you will find a decent cell phone and service that fits your description (cheap, long battery-life, and great coverage).

Here's what I have ...

Cheap:
DH and I each have a pre-paid phone. Each phone is $15 every 90 days, so it works out to about $5 a month. If you know of a cheaper cell phone, let me know, because even at that rate, we don't use enough minutes on our phones.

Battery Life:
Our battery life is okay, but not great. However, I think that depending on a long battery life for leaving a cell phone in a car is a bad idea anyway. Even if you keep the phone turned off, you will have to eventually recharge the battery, and it is easy to forget if it isn't part of your normal routine. Plus, an "almost dead" battery is just about as bad as a dead battery, because once you start talking on the phone the battery is used much faster. Some alternatives are the one-time use "battery cards" that you can use to power a cell phone with a low battery. There are also hand crank devices for powering a cell phone. Another alternative is just get the car charger for you phone (this is what we have). After all, if you're going to keep the phone in the car, you can always power it that way.

Coverage:
I can't help you there. Our coverage sucks. However, we live within a few blocks of a major freeway, and I almost never travel out of suburbia. However, I have noticed that over the years coverage has gotten better. One advantage of a network with nationwide coverage is that if you end up having to evacuate to a distant (urban) location, hopefully you will have cell phone coverage at your new location.

Special features:
You state that you aren't interested in any special features other than making phone calls. However, I would recommend making sure that you get a phone that can text message. In the event of a large disaster, you might be able to get a text message through when the lines are too busy for phone calls.

Hope this was informative.

-Kuovonne