Paying that $15 campground fee was the smartest move you made because it keeps you legal and gives you bathrooms and a fire ring, which matters way more than people admit. I slept in my Civic at Death Valley three nights and the biggest thing I learned is to crack two windows with those mesh bug screens so you don't wake up in a fog of your own breath. A cheap foam pad from Walmart plus a sleeping bag rated for way colder than you expect makes a huge difference, since desert nights drop fast once the sun is gone. Bring a small cooler and a jug of water, because you save a ton not buying breakfast in town every morning. If you're near a campground with a picnic table, use it to cook and stretch out before you crawl in the back, your back will thank you. Lot of people spend hundreds on a room they barely use, and you got the Milky Way out of the deal instead.