Traveling with a food allergy is an art. They key is to know safe places and products you can always rely on in a pinch, especially when you can't pack very much of your own food.
For example, I can eat Fritos. Fritos have saved my from hunger in more than one travel situation. They can be found in almost any gas station, anywhere in the US. They are also pretty much the only packaged food besides nuts that is available roadside.
Here's an article with some more tips:
http://summer.about.com/od/summersafetyhealth/tp/Traveling-with-Food-Allergies.htm
