Here is the description of the campground from HipCamp.com.
If Joshua Tree camping is on the mind, staying at Jumbo Rocks campground is a must! Family-friendly Jumbo Rocks campground is located just a short hike from Skull Rock, one of the coolest rock formations in the park. Jumbo Rocks campground is the biggest of all campgrounds in Joshua Tree National Park. Pick from 124 first-come, first-served sites that have picnic tables, fire rings, and pit toilets. Potable water is not a luxury at this campground, make sure to bring plenty, especially in the summer months!
Jumbo Rocks campground usually fills on weekend nights October through May and can be especially busy during holiday months. Remember, advance reservations are not accepted here! Try arriving on a weekday to have the best chance at grabbing a first-come, first-served site at this acclaimed campground.