The above comments seem to be our experience also. The only time in the past 5 years that we were unable to get a space without a reservation was about 5 years ago when there was no opening in the RV parks around Jackson Hole that night. I remember that day we were near Montpelier, Idaho when we called the Jackson Hole area parks. Under the circumstances we Just pulled into the nice RV park just outside of Montpelier, stayed the night and went on to Jackson the next morning.
Normally (almost always) we just determine where we will be in the evening while we travel and about 3PM, or so, we call the RV Park where we would like to stop. This gives us assurance that there will be a space for us within a couple of hours and, as a I mentioned, it has been about 5 years since we didn't find an opening where we called.
