Cost of food while camping ... does it have to be high?

I've found for short trips, I can stock the RV with food from home, and it doesn't cost extra, since I was buying that food anyway.

On longer trips, when I'm away from my home grocery, the mom'n pop grocery just outside the campground usually costs double on groceries.

Sure I could go on a drive (sometimes a long drive) for a larger discount grocery, but the time cost outweighs what I'd save on food.