Temperatures in Zion National Park are consistently warm, even through the winter, and many of the sites are open year round as a result. Just note that there are no shower or laundry facilities at the campgrounds within the park, so be sure to plan ahead to account for this.
To beat the crowds and summer heat, I recommend camping in Zion in either early spring or late fall - you'll still see all the sites, but you'll have a better chance at nabbing a camping spot and be way more comfortable at the same time.
Lava Point Campground
Lava Point Campground is a first-come, first-serve site that is far away from the crowds. An hour and a half drive from the South Entrance, Lava Point is a primitive campground and is free as a result. You'll find pit toilets and trash cans here, but no water or other amenities. Definitely stock up on food and water before you set up shop.
If you want to make some friends, have tons of amenities, and be in the center of all the Zion action, then Watchman Campground is the place to be. Not only are there 63 RV sites, 66 tent sites, 6 group campsites, and a visitors center, there is a nearby shuttle stop to get around the park, amazing views of the surrounding mountains, and wheelchair accessible sites. Campsites include amenities such as electrical hookups, flush toilets, fire pits, drinking water, and a dump station.
However, Watchman is probably the most popular campground in Zion so be sure to make your reservations early. The campground is open all year, and if you’re visiting from March through November, you can make reservations up to six months in advance.
Just down the road from Watchman, about a half-mile from the south entrance, South Campground is another popular spot in which to camp in Zion. Each site here is large and comes with fire grates, picnic tables, and drinking water access. You can make reservations for campsites here up to two weeks prior to your arrival date.
South Campground is open from late February through late November, and sites are first-come, first serve. Although there aren’t any hookups or washrooms here, there is a dump station and potable water.
If you’re looking to get away from the crowds and camp on your own, then heading into the Zion National Park backcountry is the way to go. There are a few great trails to camp on, including the West Rim Trail (which has 9 campsites), Orderville Canyon (a Canyoneering hotspot), and Deertrap Mountain.