Part of the issue is where you end up buying the food --- gas station mini-marts aren't particularly cheap in general, but sometimes that's the sort of location available to you. Still, things are less expensive in the south in general. Buying along the way is IMO generally a better way to go unless a person has particular dietary issues. More flexible, and postage can eat up some or all of your food cost savings.

Have a great time, and I hope you get a lot of bang for your (300) buck out of the trip! :-)