Be particularly careful if you're passing through "non gun friendly" states. From the reading and news stories I've seen with regard to the matter in some places, they consider any stop (including stopping to eat or fuel up) as a violation of the law, since you're not passing through at the particular moment, you've stopped.

I have no personal experience with it, but have seen enough "horror stories" of such places that I would definitely suggest being careful about planning stops carefully.