There are campgrounds outside the park on the north side along hwy 62. Check in 29 Palms, Yucca Valley or Joshua Tree. My favorite CG inside the Park is Jumbo Rocks. No water!!!
It is hard to believe but when the government took over the campground on the west side of the park they removed the hook-ups. None of the campgrounds have full hook-up, some have water.
There isn't any hookups in the park. There a couple of RV parks on the north side outside the park. Very few national parks have campgrounds with hookups.
The closest to the north entrance is probably 29 Palms RV Resort (800) 874-4548.
We spent a couple of nights at 29 Palms RV Resort last month; it is a very nice place, golf course nearby. If you need hookups, then I'd go here; ask them if they have a shaded site available when you check in. We camped at several CG's in the park also, Jumbo Rocks was the best even though there were no hookups or water. If you're self-contained, I'd recommend that you try a CG in the park and enjoy all the sights and things it has to offer. Stay away from Black Rock CG though, it's in pretty poor shape, at least compared to the other NPS CG's in the park.