Camp grounds can run anywhere from about $15 for government operated, no hookup, usually in the west, to over $100 for some resorts. Usually $30- $50 for full hookup sites. Walmart is free if you stay out of the store.
Gas mileage will run from 7-8 mpg for larger Class Cs to about 15 for newer smaller units.
Repairs can be expensive. If you are a DIY person you can save big.
Meals, well, If you eat in restaurants a lot expect to spend $$$. Eating in may cost a little more than at home since you can't take advantage of the sales you would at home.
Figure it out for your camping style.