We loved Snow Canyon State Park right outside of St. George, which we visited in November -- video here:
https://youtu.be/Ae-XLFXVdk4. Many sites have electric/water and there's a community RV dump on the loop. Be careful of which site you choose, however, for 1-14 are insanely close together. We had 15A (W/E site) which is incredibly spacious. Realize that Cedar City is quite a bit higher in elevation, and therefore much colder and more prone to snow.
And if you prefer to boondock there are great spots near Virgin:
https://youtu.be/O_CeDJtVs0s