Having lived in Joshua Tree since 1970 and explored just about every square inch of the park, I'd say that there's great stuff to photograph almost everywhere, and no particular areas of concentrated "good stuff". Hidden Valley and Jumbo Rocks are both nice campgrounds, but no matter where you camp you'll surely want to explore a little first and hike a bit to decide your optimal places to shoot. You may want to drive somewhere for your sunrise and sunset shots if you have enough time to explore for what you like best. You probably won't find exactly what you want shooting from your sleeping bag out your tent flap or RV window.
Too bad you're not here now. I looked out of my window a little while ago and was shocked to see several inches of snow on the ground and still plenty coming down. We almost never get snow right at my house, so it should be fantastic in the park once the sky clears. That's my consolation prize for not being able to work on my camper project for a while now.