Another tip that we've learned is that it's smart to stop by a grocery store on your travels rather than grabbing fast food or going out to eat. This isn't just for when you're camping or staying in a hotel and able to cook, either.
The deli section at your average grocery store is a great choice for affordable (and often healthier) food. The subs are usually much cheaper than Subway and are good. Pre-made salads and such often come with the dressing and a fork included. 8-piece fried chickens are usually much cheaper than KFC and the like. Wraps, popcorn chicken, etc. are often available, too. Plus -- my favorite -- is the fresh-cut fruit. It's obviously not as cheap as buying the fruit yourself and cutting it up, but it's still a healthy choice.
Plus, while you're there, you can stock up on other things for the road. Buying chips, granola bars, snack cakes and other snacks at the grocery store is much cheaper than grabbing them at a convenience store.
It does take a little longer than running through a drive-through or stopping at a convenience store, but it doesn't take that much longer. It is a good way to stretch your legs, at least, and you can save a LOT throughout the course of an average road trip if you make your question, "Where's the nearest grocery store?" vs. "Where's the nearest McDonald's?" You might not pack on as many pounds, either. :)