You are going to want to purchase a good Truckers atlas. You are also going to want to have a way to keep track of your expenses on the road. An expandible folder works wonders for this. That way you have one place you put all your receipts. I would also pack a few towels. If you end up running on the turnpike in Ohio, the newer ones have free showers for drivers.
As for how many pairs of socks and pants for a 3 week time. I know when I pack my husbands bag for him to go out on the road. I make sure he has 6 jeans 6 shirts 6 socks and 6 pairs of underware. This way if something happens he can have extras. Always have a jacket handy. You might be in Georgia Monday, but Tuesday you could be in Ohio, and it could be cool out.
Packing the way that you are packing you are going to have to wash clothes while you are out on the road. It is cheaper to buy laundry soap at home then in a truck stop. That or find a Wal-Mart or K-Mart. One of those plastic zip lock containers works wonders for storage of the soap. If you leave it in the box you run into the chance of it tipping over, and if it is in a plastic bag, there is a possibility of the bag tearing. You are also going to want a dirty clothes bag.
You said carry some food. Make sure you have bottled water too. If you are somewhere that you don't have facilities, you can always take the time to brush your teeth. Baby wipes help with a quick clean up. That way you have a fresh feeling incase you can't make it to a truck stop to take a shower.
This is coming from a recruiter and a wife. I know it is the tip of the iceberg, but I am sure others will fill in the holes I left.
