I've visited Zion a couple of times and have stayed at the South Campground. This trip I stayed at the Watchman Campground. We arrived into Zion around mid-day and immediately hit the trails.
I hiked the Angels Landing Trail while my friend hiked some other trails. We finished at around 4pm then drove into town for a bite to eat. We ended up eating at the Thai Sapa Restaurant right outside the park in the nearby town of Springdale.
I knew that there were not showers within Zion NP so I inquired where I might be able to find a pay shower within Springdale. I was directed to the nearby Zion Outfitters Store. Attached to the store there is a shower room and laundry room that anyone can use. The showers operate on tokens that you can purchase from a machine and cost $4 for about 5 minutes. After showering, we ventured back into Zion NP to find a campground.
We were camping in the winter time, so the park was pretty empty and we had our pick of campsites. We drove around the grounds and found a spot close to the restroom. The restrooms do have sinks and toilets, but no showers. We were camping in a campervan, so setting up camp was easy. We simply pulled into a space, paid our camping fee at the after hours kiosk and we were all good.
We didn't really have a chance to explore the campground much. But the real advantage of this campground is that it is conveniently located within the park. During Spring, Summer and Fall you can access the free shuttle. And it is close to the exit of the park so that you can easily travel to the nearby town of Springdale to pick up any supplies, shower or do laundry.