We boondocked in the overflow camping area just outside the entrance to the park cloest to the I10 and Cottonwood campground. This area is free and easy to get in and out of, and only four miles from the park. I must have seen 75 camping spaces with decent room in between. Just turn on Cottonwood road from I10 and head northeast; in about a mile you will see a dirt road that goes in both directions. Turn either way and you have a vast desert to camp for free for a 14 day stay limit, though I saw no official checking the sites over the weekend. Just find an open fire pit and set up camp.
There is a really convenient spot for stopping for the night just off I10 to the west. Looks a little trashy but will do for a quick overnight or to rest up. The coordinates for this spot are 33.65731 -115.80208
Further down the road is Box Canyon with 15 miles of nice camping areas. Though the sand might keep you from some of them.
Then on to a great find. The General Patton Museum. Not only a nice visit back into our American heritage but FREE CAMPING! Just past the musuem you will see a sign "Free Dry Camping" - follow the directions to the next sign about 250 feet and see the second sign. Once there you will find 25 dry camping sites for most size rigs, though I would do a walk through if you have a very large rig. There is a turn around at the end of the dirt road.
This is a sample site with a fire ring in the rear.