We stayed there a few weeks ago and were not impressed with the campground or the ghost town. The sites in the campground are close together and not very level.
The ghost town is mostly shops and isn't very authentic but you get in free if you're camping. There are addition charges if you want to ride the little train, pan for gold or go into the mystery shack or the mine mockup.
But a lot of other people liked it - RV Park Reviews -
http://www.rvparkreviews.com/regions/california/yermo/calico-ghost-town-campground