Buy groceries and cook along the way. I often go on long (3 -7) days with my old car buddies. DW gave me this aid for Xmas a few years ago. Not only frugal, but better tasting too.
Working on oil and gas projects in Wyoming in the middle of nowhere, we cooked lots of food on our truck engines.
A part of family lore is Dad cooking a chicken on the manifold driving from DC to Tyrone, PA where his parents lived. According to the story his mother, not being sure it was thoroughly cooked, was reluctant to eat it until it had passed a careful inspection.