I was up in Onion Valley last weekend. The campground at the trailhead is official closed due to the water tank that supplies water to the campground being destroyed during the winter. Also, there was some erosion at several campsites.
Now, while up there, I was told by several hikers that the forest service allows a one night stay, must be out in the morning. There were people in the campground camping while I was there.
I suspect that the forest service would like to keep the campground only available for one night camps.
The campground at the lower level, upper and lower grey's meadow is open, and has water.
paul