I visited 11/10/2018 and as of 9/30/2018 box canyon road has been closed due to road damage. Box canyon campsite is unreachable.

Despite the posted warnings about road conditions, the drive from the main highway to the canyon was in fair condition when we were there in December 2015.
On the way through, we noticed a number of possible camping areas, but we parked right near the parking lot that visitors can use if they are taking a walk into the very impressive Box Canyon trail. There was ample room for our trailer on the side of the parking lot and we were not intruding on anyone.
As it was a Saturday, there were a lot of day visitors, but also a school group had set up camp in front of the parking area so that they could spend the night in their tents and take an early morning hike on the circular trail through the canyon.
On the whole, everybody was well behaved and friendly. However, the big drawback of tenting in this location is the lack of restroom facilities. This meant that plus or minus twenty people had to 'do their business' behind any available bush . So, when we took a walk around the area, we were constantly confronted with foul smells and visions that included used toilet paper and fresh feces.

Nonetheless, the location was just fantastic and as long as you visit during the week, you are sure to have peace and serenity most of the time.
The trail through the Box Canyon is highly recommended, even for day campers who may be a bit spooked about staying in a canyon that is vulnerable to flash flooding, especially as there is no means of communication with the outside world once you are there.

Needless to say, there was no cell signal whatsoever and there was no television signal. Our solar panels had a bit of sunlight around midday, but the shadows from the canyon quickly came over the trailer and we could no longer charge the batteries. This was one of the few times we happily used our generator, especially as there were no other campers on our second night here.
Favorite things: the walk through the canyon and also, in the opposite direction, along the dry river bed. Very surreal setting.
Negative: No restrooms! Human feces and toilet paper on every bush in the immediate vicinity.
No cell or television signal