I've overnighted several times like that. There is a campground that I stayed at a couple of times where the office was only open 10-2. we got in around 9pm and left by 8 am (didn't even unhook). We were just looking for a stop, with electrical hookups to run the AC, to make the next day's travel a little less. They have late checkin procedures, and after getting to the site, I went to the office, filled out the paperwork and left a check, per their after hours instruction.
There was one time at a different campground that we had to track down the host to pay, because we wanted to get on the road earlier than he made his rounds, and they had no self pay procedures.