Fruit and vegetables are vanishingly scarce once you get away from the coast. That doesn't leave much besides cereal, confectionary, crackers and cheese.
I found fruit and vegetables difficult to find on the TransAm in Virginia and Kentucky, partially because of the fact that the TransAm avoids all the cities large enough to have grocery stores. But finding fruit and vegetables was not a problem from Illinois through Oregon, nor on most of the Northern Tier.
I buy a lot of apples, bananas, oranges, carrots, lettuce, mushrooms, cucumbers, raisins, etc. and other fruits (blueberries, pears, peaches, strawberries, etc.) where available. If you can't find fresh fruits, you can settle for canned fruit. Bread products, tortillas, bagels, nuts, peanut butter, chips, ready-to-eat or made-to-order sandwiches (veggie or not). Even perishables will keep okay for half a day in your panniers. I typically bury chocolate milk or orange juice in the middle of my clothes to insulate them if it's a hot day. You can get a small box of cereal from the cereal aisle and discard the cardboard box.
Your options increase if you are an omnivore. Canned stews can be eaten cold. Beef jerky (a bit pricey for my taste most of the time), canned beans, canned tuna, deli meats, fried chicken. If I can find a Subway sandwich shop, I often get a foot-long, eating half for lunch and half for dinner. Many grocery stores and even some gas stations can also make sandwiches to order. Most grocery stores have premade sandwiches. A 2-pound $8 sandwich from Safeway provides me the better part of three meals.
Even if you are in an area where grocery stores are few and far between, make your most of the opportunity when you find one.