I've never heard of "early check-in" ... you would have to pay for the night before, I would think.
Sometimes you do get lucky and the site they have assigned will be empty when you arrive and you will be able to set up early. We have not had this kind of "luck" in several years though. We have had to wait until 1:00 before they will give us our site number. Because you park at the front after check-in and before you have your site number (if you arrive early), you can still enjoy the amenities. We have walked around and seen the sites that were empty but not "cleaned" ... our's was usually empty but not released by housekeeping !!! Grrr!! In fact, last year it never did get cleaned ... they got tired of me pestering them and then at 1:00 when they were required to release it, it still wasn't clean so they acted like it was suddenly cleaned/available. I knew better from the way it looked but was just glad to have it available for set-up.
I don't know what their system is for site cleaning, but I don't think they have as much staff as they used to and so the sites aren't getting cleaned up prior to 1:00 like they used to.