This mile wide strip of land between I-10 and the entrance to Joshua Tree National Park is a patchwork of private and BLM land. A dirt road provides access to 10-20 good boondocking spots. Most are fairly level and large enough for any RV.
The park service promotes this spot as a good place to camp if the park campgrounds are full so don’t expect complete solitude. It’s also easy to access from I-10 which makes it a nice place to overnight when heading west or east on the interstate.