$10-$15 a day for food, and drinks is very doable. As a long haul driver. I have been living on that budget since 2007.
My normal schedule would look something like a breakfast bar in the morning. For lunch I would keep it light with a sandwich, or hotdogs. In the evening I would have something on the order of a subway sub. Maybe every 2nd, or 3rd day I would stop to find a good sit down restaurant and have a real meal. I try to stay away from fast food as much as possible, but is does help in a pinch. I find that snack type foods (snickers, payday, jerky etc...) help considerably to reduce the effect when I get hungry.
Sometimes buying in a larger size is better. Especially when it come to drinks. A 1 gal jug of water is around $2.00-$2.50 in most places. The same amount in 1 liter bottles would double that price easily. Refills are another place to save money. A 24oz coffee is around $1.70-$1.80. I get 2-1qt stanley coffee thermoses filled for about the same price.
One place to steer clear of when eating is the major truckstop chains (TA, Petro, Pilot, ETC...). You will easily blow your budget for a couple days when eating at one of these places. If there is a truckstop. There will be a grocery store not to far away.