We were hoping not to have to live with reservations in Nat'l Parks, but our experience says otherwise. Last year, September, after Labor Day for a month, we were unable to get a spot in Yellowstone, Badlands or Glacier. Part of the problem for us was they start closing campgrounds mid-late Sept in some national parks.
But a classic example was Badlands - we were a day early for our reservation in Custer State Park. I called the campground in Badlands Nat'l Park at 9 AM. On the phone they told us "no problem, campground is wide open, you won't have a problem for tonight". We arrived at 4 PM to a sold out campground and the rangers then told us the campground had been full every day by just after noon. She had no idea why the person on the phone told us it would be wide open. Ugh.
Lesson learned - don't drive all day hoping to get a first come,first serve spot. Or plan for the first come first served dance as outlined above by Artum Snowbird - we've experienced this is several nat'l park campgrounds -- especially Capitol Reef.
Had a similar experience with Big Bend Nat'l Park in late March ... then the problem was spring break. It seems it's always something....
Not as easy to wander as we had hoped.... at least if you want to stay in a park not a commercial campground.
