Joshua Tree NP in the spring can be like a zoo gone wild.
The two NP camprounds that accept reservations, (Black Rock & Indian Cove) while inside the park itself, are isolated and are really not connected to the rest of the park. All of the campgrounds inside the park proper are first come-first served and have no hook-ups and NO WATER whatsoever. The only water is at the entrance stations, check when you enter. Here is a link:
We were there about two weeks ago and stayed at Jumbo Rocks c/g and it was less than 20% occupied, but that will change as soon as the first of the spring flowers appear. Some of the in-park camp sites ARE NOT suited for trailers of any length.