You can get an idea of gasoline costs by going to
AAA Fuel Cost Calculator . You can use the closet U.S. city as your starting point and then add a few dollars more to get an idea of how much it will cost you to travel to Orlando.
I agree that eating 2 meals in the camper will save you a ton of money. But I would encourage you to do breakfast and dinner there and have lunch in the parks (lunch will be cheaper in the parks than dinner will be). My DSis camped at FW a couple of years ago and her biggest regret was that her DH insisted on returning to the camper for all of the meals. By the time they got back to the camper, she prepared the meal, they ate and then cleaned up, they spend close to 4 hours of prime park time doing lunch. Since you're going during value season, the park hours will be shorter anyway.
Take your crockpot along and do dinners in it before you leave for the parks. Add some salad & bread and you'll have dinner ready within minutes of returning to your campsite. Then you can return to the parks afterwards for nighttime parades and fireworks (and maybe a little dessert
) Bring what's in your pantry to complete the meals rather than going out to buy extras. Ideally, you should only need to buy milk and bread from the FW store.
I agree with the others about limiting the kids' souvenir selections to a set dollar amount. Bringing a few goodies (like the glo necklaces and light spinners) will reduce the "gimmees" a bit. Bring sunscreen with you because it will be expensive in Disney and you will definitely need it. Drink water from the fountains instead of buying bottled water (at $2.50/bottle), or ask for ice water from any of the CS places. Add mix & go Koolade or Crystal Light to it if the taste of Florida water is just unacceptable to you.
When you eat lunch in the parks, split the meals. Buy a couple of combo meals and split the a large beverage between all of you. Every park has at least one CS location that offers a great combo meal. Try the half slab of ribs w/ corn on the cob at Flametree in AK or the ½ Rotisserie Chicken w/ mashed potatoes and veggies at Cosmic Ray's in MK. If you're adventurous, the shawarma platters (either lamb or chicken) at the Tangierine Café in EPCOT are very filling. And if an adult would prefer a light meal, they can order a child's meal at any CS.
Good luck with your plans! You must be so excited!