I agree the NEXT EXIT is a good tool for planning down the road stops as your progress. I would not count on a single gas station at an exit, but if the books list four or five it is a pretty good bet there will be at least two or three still open.
Though for night planning the next day's run, I'll use my laptop with Microsoft Streets and Trips. It too will be slightly out of date (if you keep it updated) but fairly accurate. Just don't like to have the laptop up front as it's a hazard if something goes wrong. Plus the papermap is usually quicker and less distractive.
Of course, I have never worried about planning fuel stops. I keep an eye on prices and when I get near 1/2 tank I find one with reasonable prices and stop. Only once out of 75,000+ miles did I get caught and that was coming out the back side of Bakersfield. Didn't like the looks of the one station so decided to wait to another. Didn't realize the town ended so suddenly with nothing for 75 miles beyond. Figured I had plenty to make the next town which I knew was big enough for at least two stations, but still sweated it. Stopped at the first station I came to which ended up saving me 25 cents a gallon - and this was back when it was $2/gal.