There are 2 sets of walkin campsites at the trailhead. The first is called the backpacker campsites and are limited to 1 night. These sites are situated between the overflow parking and the trailhead.
There are new walkin campsites below the overflow parking. These sites were added less than 2 years ago, and are rarely used or known about. There are approximately 12 of these sites and each have a bear box and table, along with a level pad for tents. As far as I know, there are no night restrictions, except, it is not easy to carry large items to them....
While up at the Portal, Memorial Weekend, I visited these sites. No one was using them.....they are off the beaten path.
Either talk with the camphost at the family campsites, or Doug at the store since he participated in the building of the sites.
The camphost actually is in charge of the sites.
Oh, there is water also.