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A typical car will have about a 44 Amp Hour battery in it. Your headlights on low beam with running lights draw about 95 watts, or about 8 amps. Your heated seats are probably one of the larger draws, probably around 180 watts, or 15 amps. Add another 40w for the stereo, or 3 amps, another 60w for the blower motor on your heater (5 amps) and you're talking about a constant draw of 31 amps. If you're EOC'ing for 30 minutes, you've tapped out over half the capacity of the battery, then you want to use the starter, which draws 75 amps. That's where your problem lies.
That's why it's best to EOC when it's light out, with minimal accessories on. Your idle probably burns on the order of .15-.25 gallons per hour. A half hour of coasting at 30mph means you'll use .075-.125 gallons while covering 15 miles (that's still 120mpg while coasting with the engine on). In any case, if you're going to try to EOC, nix the power seats, keep the blower motor on the lowest setting, and look at converting to HID headlights and LED marker lights.
Overall, even a deep cycle would have problems with your demand. If you can find a 55ah battery, great, but you're still going to have to avoid using those heated seats, buy some high efficiency lights, and be careful with demand. I'm learning I can EOC during the summer, when the sun is up early, goes down late, and I don't need the heater or defroster on, but right about now, it's impossible. It's too dark, too cold, and I still need to be able to operate the vehicle safely and reliably.
120mpg coasting still beats the heck out of 0mpg sitting on the side of the road waiting for a tow truck.
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