When you arrive, they collect the money at a canopy at the end of the parking lot where you cross the road to get in to the camping area. You park your tow vehicle in the parking area where you pull in off the road on the right. They generally don't like cars kept in the camping area, but they are pretty tolerant of it if you are in the tent area and not where the RV power pedestals are. Hauling trailers are supposed to be parked in the field on the left as you pull in - they like to keep the main lot open for people coming to the show and campers personal cars/trucks.
If you get there that late, I'd just try to find a spot, then pay in the morning when they are out. The folks that collect the camping money stay in the big white Montana 5th wheel in the back row (if that is what they are still using). Gate usually stays open all night so you should be fine. You will have a great time