For comparison a week ago a buddy and i stopped at a family style country restaurant. We each got the dinner special which came with a salad, both got sweet teas, and i got a slice of pie. Paid with a twenty and got a 5 dollar bill back. Sure it was a normal serving size and not the huge servings like you see ay a petro or TA, but unless you want to be five hundred pounds you don't want to finish those huge platter dinners.
Edit to clarify. When i said i paid with a 20, i was talking about paying for BOTH meals.
It's all about the fridge, and a stocked pantry on board. Find a Grocery Outlet, stock up for cheap, eat healthier, and stop where you want for a nice picnic. I avoid truck stops if at all possible, and don't eat the overpriced junk.