On saving money on the food budget:
My husband and I always carry a small electric grill or frying pan, to use in motels. It is a great way to cook up a couple of hot dogs, grilled cheese sandwiches, hamburgers or grilled chicken, and so forth.
A cooler is part of our gear, too. It carries our drinks, purchased at a big-box store or our local grocery store (whomever had the sale on them). When we run out while on the trip, we stop at another big-box store and get more. As Lifey suggested, those convenience stores and fuel outlets will cost you so much more on your drinks and snacks. Cooler can also carry a few fresh fruits and vegies for munching on. One word of caution: motels don't usually like you to fill a huge cooler with ice, because it eats into their ice supply for the day. However, if your cooler is small enough (say, a 6-pack cooler size), you can probably get away with it. I just discreetly fill the motel's ice bucket a couple of times and walk it back to the room, rather than blatantly filling my cooler. If your cooler is much bigger than that, plan to buy ice every other day and budget about $80 for ice. (That's what we used on a 6-week trip with a large cooler.)
Sometimes you'll pull into a motel and find out that the room has a microwave oven. More often than not, these days, the rooms have refrigerators. Microwaves are great - you can go to the store and pick up a frozen meal to heat up. BTW, you'll want to carry some silverware with you; eating from plastic forks for 6 weeks gets old.
Our last long trip was about 8900 miles, took almost 7 weeks. However, we are TWO adults, and did only a little cooking in our rooms. This is our expense report. There are some things in there that might surprise your budget, like souvenirs, and the fact that my husband and I both love ice cream and stopped a lot for that. (Something that isn't possible to carry in your cooler.) This is the trip report itself. Note that we did not purchase the Annual Pass for the national parks last year. Before our trip, I added up the entrance fees to every park we were going to go to, and the cost was lower than the Annual Pass.
Donna