I really know what you are talking about! When I was doing triathlon training earlier in the year I ran into this problem. Here's what helped me:
1. Stay hydrated--are you drinking enough water or gatorade type fluids t/o the camp?
2. When you're exercising you don't want to eat something big and bulky, you need something quick and easy to digest that takes the edge off and keeps up your energy--especially for something over 3 hours. I really did well with eating a banana, Clif Shot Bloks are awesome (gummy electrolyte chews that do well in your pocket, etc) or gels. I know some people don't like the gels, but they have saved me a few times during workouts.
3. Keep a cooler in your car with ice and healthy snacks that are easy to eat for right after your workout. Banana, cut up oranges or other fruit, protien bars, pretzels, dried fruit, nuts, protein shakes, etc.