Depending on where you are, some campgrounds have community bear vaults that you can store food, etc. in. When I was in the middle-of-nowhere Montana, it was also acceptable to keep food in the car (but they recommended the trunk, and even better if it was in something like a well-sealed cooler in the trunk). Some places have bear canisters for rent, too. So if you're staying in or near a National Forest, I'd start by just calling the local ranger station.