We thought, due to the down economy, that last minute planning might be ok for an August trip to Yellowstone in our 25' Airstream trailer plus tow vehicle since we don't care if hookups are available - NOT.
We were planning to stay about a week.

All NPS campgrounds in the park that take reservations are pretty well booked. We wanted to avoid the commute in everyday from W Yellowstone or Gardiner, but I am researching options there now.
I got 2 nights at West Thumb, that was the only thing open. We were hoping for Madison.
Does anyone have experience with cancellations - how likely? Or first come first serve at this time of year?

Of course I can try calling every morning to see what they've got. The problem seems to be all the sites can be reserved with only 2 day cancellation notice, so notice if any will be very late.
Thanks for any assistance.

I recently saw a report that visitor attendance in Yellowstone NP is up by just over 20% over last year. They say that most people are now staying longer when they come to the park.

If you do not have reservations in the park, there might be some spots available if you are at the park before noon on any given day. But it will be a gamble and you won't know for sure until you get there, for a first come, first served spot. I would not plan to stay in the park if I had no reservation.

Thanks for that interesting tidbit. I was able to get a reservation today at Grant (west thumb) for Aug 6-13 like I wanted by requesting two sites - one for the trailer and one for the car.

Car plus trailer are 42' long, I had a feeling that we were being limited by that (the YNP website claims limited sites over 40').
You have to sleep on each site so we will pitch a tent on the car site, the kids we'll probably go for this.