Regarding Yellowstone - if you can zone in on your dates, suggest you consider going ahead and make reservations for inside the park full hook-up. They sell out early. We were there for 4 nights in 2008 and stayed in a "non-hookup" camping area. We found their generator restrictions were quite confining to our life style and thus our batteries ran down. Had to power the slides in via hand.
In my opinion, staying inside the park is a lot more convenient, time wise, than the outside campgrounds. But you do have the issue mentioned above.
We were on the road 10 weeks, traveled 9,200 miles and had no other planned reservations and had no issues finding a spot. To conserve cash, we do stay in Wal-Mart about 1/3 to 1/2 half the time but we always phone ahead for permission before stopping.
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