I'd suggest all of the above -- everything everybody else suggested is something I would have thought of (and do before driving a few hundred miles, let alone thousands).
A $10 90 watt inverter will charge and run your laptop using its AC adapter. Download Google Maps, and if you can, get Microsoft Streets & Trips (one of Microsoft's best acquisitions, and after some years they still haven't ruined it). The only thing better than a laptop with Streets & Trips is a decent GPS, and best is to have both.
If you can spend $16, WalMart sells a 220 watt inverter that will run your laptop and a few external hard drives.
http://www.walmart.com/catalog/produ...uct_id=9224432
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