Most state parks we've been to have a locking gate with either a combination lock or electronic access. The code is handed out at check in. You might be able to call ahead and get the code.
That being said, when we had a travel trailer, we did late arrivals a lot, usually at KOAs since most have well documented after hours arrival procedures and most have Pull Thrus. I wouldn't want to wake people up trying to back into a site at night. Now with the Motorhome, we usually just stop at Wal-mart and head to the campground in the morning.
-Michael