The Diablo Drop-Off looked really challenging going up. Going down after dark was no easy trip either! Going up there's a big patch of loose "sugar sand" that is deep, long, and pitched at a pretty good angle. We watched a bunch of trucks - Range Rovers IIRC - struggling to get their last couple of trucks up.
The State Park maps and the guide books all consider Diablo Drop-Off a one-way trail, downhill towards Fish Creek.
Who is thinking of camping? There are several options, and I may be up for camping as well. Keep in mind that inside the State Park boundaries you must have any fire in a metal container, no ground fires allowed. You can, however, camp pretty much anywhere as long as it's not on a trail and at least 100' from any water. Pack out all trash, of course.
Fish Creek is pretty wide most of the way, and there are spots along the way that look like they would be good. Some of them look like we could get quite a few trucks in. Sandstone Canyon gets really narrow. Most of the spots I've seen back there would be good for only 2 to 3 trucks, and then not that far apart. It's hard to say how crowded it will be, last year there were a lot of people, and we spotted more than a few campers. The whole area is rather active seismically, so you'll want to avoid parking under any cliffs or rock walls. Ask Jason how much a new rear hatch glass costs.