You'll have plenty of room, no matter when you arrive. This show/meet doesn't draw the big crowds like it used to 10-15 years ago.
I have taken our 42' motor home to York many times and have always found a place to park. I typically don't get there until around 3 pm on Wednesday and there are still plenty of sites at that time. There must be a hundred sites scattered around the grounds. Check in behind the grandstands with Dave Goodheart or one of his assistants.
30-amp electric is available on all the sites, but only a few on the north side of the track have water and none have sewer hookups. Best to arrive with your water tank full. There is a single dump station over at the west side of the grounds if you need to dump the tanks before you head home.