I'd call those campgrounds asap and see what their cancellation fees (if any) are. Most of us don't make reservations but just call ahead to see if they have any openings. The only exception I make is if I'm going to be in a park on a holiday weekend.
If you're going to crank out the big miles the first few days, do what several others here have suggested. Find a place to legally park and catch some shut eye. There are lots of posts on the forum about free places for the night. Some Walmarts, Cabelas, Bass Pro Shops, truck stops, rest areas, etc. are possiblities.