well if you and your mom are creative economic cooks you can do a lot to keep your food budget down while camping at the fort for your WDW stay. sit down ahead of time and come up with some meal ideas and a shopping list for each meal. some items you could buy at home when you see a good sale price, pack and bring with you and other items you could pick up on your arrival day.
since you are camping i am assuming that you will be driving down. on your arrival day, swing by a local grocery store and stock up on snacks, fruit, drinks, the items you need for the menu i mentioned above.
if you have a fridge in your camper you can keep items cold while at your campsite. if not, perhaps bring along a cooler or buy an inexpensive one at your grocery stop once in Fla.
if you have a small gas grill and a crock pot that you can throw in your camper, that opens up tons of good, cheap campsite meal options.
We (then a family of 3, two adults and one 10 year old who eats like a man) have camped there for 11 days and with occasional eating out and using the campsite eating tips above were able to eat for around $100/day. that is an adverage. on days where we ate at say a character meal or an upscale sit down meal we went way over. some days when we ate mostly at (or from - as we would pack lunch to take with us) the campsite we were way under.