Zion National Park has two large campgrounds near the south entrance to the park, Watchman Campground and South Campground. Only Watchman takes reservations, and only Watchman has any sites with RV hook ups (electric only). From a location standpoint, South Campground shares the features of Watchman: they are both a very short walk from the visitor center and the shuttles up into the canyon and down into the town of Springdale. The shuttle system is a marvelous alternative to using a car unless you are leaving the canyon.
Watchman Campground is set up in six sections: A and B loops (70 sites with electric hookups for tents or RVs), C and D loops for tents only, E loop with seven group areas, and F loop with 18 walk-in sites and a parking lot. The A and B sites are larger and closer to the Virgin River, with many sites right on the river or with nearby access. The C and D sites are close together with little privacy. They also tend to have a younger population and be a bit noisier. All the campgrounds are in high demand for most of the year and are generally full, but there is a lot of turnover.
The views of the surrounding peaks and canyon walls are splendid from nearly any campsite. Several trails leave directly from the campgrounds, and the shuttles take visitors to all the main trailheads with almost no wait between buses. The hiking in Zion is spectacular: Angel's Landing, The Narrows, Emerald Pools Trail, and Lower Subway are all unique hikes through a stunning landscape. On the other end of the spectrum, the campgrounds are also a short walk or shuttle ride to coffee shops, pubs, restaurants and services in town. Few places have this "best-of-both-worlds" situation.
Watchman Campground is open all year (some loops may be closed from time to time), and reservations are only available from March through November.