Is this the same trip you wrote about, a few months ago? If so, a moderator will probably merge the two threads. That said....
I just finished budgeting for our own trip coming up, which is about that long in terms of mileage and time. The way I do it -- I estimate $75/day for travel days, $50/day for non travel days, for food for two. We do some eating out, but we use a cooler for lunches and eat from grocery stores, plus we tote some cooking equipment to use whenever possible. Don't forget to budget for ice ($2/day), as many motels will not let you fill your huge ice chest with ice from their machines.
For lodging, like you, that's pretty easy to figure out. Same for fuel.
As for sightseeing and souvenirs -- I figure out all park and museum entrance fees, boat tour prices, etc., in advance. Souvenirs, though, will depend a lot upon what you plan to purchase. On our last trip, we picked up cheap t-shirts at most places, for $10 each. If we saw a book that we really wanted to read, in a gift shop, we usually wrote down the title and author, and then checked it out of our local library later, or ordered it from Amazon. (The one exception was a road guide, which was needed that day, but that was $5 or so.)