29 Palms has a nice RV park, but inside the park is all dry camping and except for Cottonwood, probably has few spaces for such a large rig Between Yucca Valley and the town of Joshua Tree is Indian Cove Campground and would probably accommodate you, its also dry.
Even being dry, it is often filled up as it is a World renowned rock climbing area so very popular with the climbing crowd. Cottonwood has a nice long hike to an oasis and filled with spring flowers, but its miles from the "action".
Make sure you take the Ranch tour, (check in the Visitors Center in either Joshua Tree or 29 Palms, (BTW both are worth visiting) and if you like semi easy hikes, the Barker Dam hike and the Hidden Valley Hike are fun. BUT be sure to take lots of water.