Yellowstone is a massive park, so if you can stay in the park, it will save you time that you would otherwise be spending just getting in and out. Some of the lodges and reservable campgrounds are filled up a year in advance during the busiest times of the year. If you’re visiting in the winter, only Old Faithful Snow Lodge and Mammoth Hot Springs Hotel are open. Most are open from late Spring to Fall, with the exceptions of Mammoth Campground that is open year round. If you’re looking for full hook-ups, Fishing Bridge RV Park is the only site that offers that.