Picnic style sandwiches.
Ideas include eggs, tuna, hummus, falafel, whatever it is that you love. Stopping for a picnic can help break up a long day of driving as well as being simple to make and cheap to buy. You could have bread, bagels or wraps and pick up some packets of condiments from a fast food outlet or garage. Carrots are great for salad as they withstand being out of the fridge, can also be used in dinners and they are cheap to buy. Adding a tin of beets to the mix makes sandwiches more adventurous. Cucumber lasts long enough to keep in a cool box and adds a bit of green. Eating a cold lunch saves using gas for the cooker, if eggs take your fancy boil them the night before in the same pan as your pasta/noodles.
Soup. Uses gas but there are so many tinned varieties and it’s quick to cook then snack on with a bagel.