There is a backcountry pit toilet at the primitive campground near Split Mountain. It was a very timely discovery for me, LOL!
I think it would be worth mentioning the town of Borrego Springs, which is entirely surrounded by the park. The town includes several restaurants, hotels/motels, and gas stations, though it can be pricey there. I definitely recommend Kendall's Cafe to eat (accessed from the south side of the town's shopping mall). I'm told that the locals prefer to keep that one under the radar so it doesn't become over-crowded with tourists.
Also, my guide during my recent four day trip to photograph the area (a 15 year volunteer at the park), told me that the park's lowest point is one hundred something feet below sea level. Sorry, I don't recall the exact value (but could get it for you by contacting him).
Finally, there is an excellent (but non-topo) map of the park useful for any recreational activities. It is available with and without an associated guide book.