The website says all furnace creek campsites are reservable and none are first come first serve. Are there ever tent sites that are available on a walk in basis?

For our visit to the campground in February; all sites were reserved months in advance,

In wintertime, it is reservable only, and in summertime, it is first come first basis. But if any site is empty and not reserved for any day, they generally put note on that site# so that anyone can come and stay with pay.