Last year I arrived with Evil Genius and we were able to get signed in and access to the paddock at about 5-6 pm if I remember correctly. There was plenty of room to park. Then the unloading and beer drinking began. Plenty of beer was stashed in plenty of trailers that no one went dry. Last year only the EAST track was used. Very fun track.
Here is the track map http://www.millermotorsportspark.com/track-maps.html
Running the East track, the access ON and OFF the track was at the WEST end of the Pit lane. See that little black line over the gray area just to the left (east) of the ACES cafe. That is the ON/OF road to the pit lane. To get on the track you hang a right onto pit lane, then a left on the access road bypassing turn 1. Then a Left onto the un-used portion of the track, wait for an open spot, and hit it, heading directly towards turn 2
The West track was being used on Sunday by a motorcycle track day group, and if I remember correctly the track ON/OFF was somewhere near the Exit of turn West Ten.
So paddock parking will be best somewhere in the vicinity of the First Aid station, radiating outward from there.
P-box was in the East Day garage.
Dudes Ex Machina:
https://www.facebook.com/dudesexmachina?Everyone who has ever built anywhere a 'new heaven' first found the power thereto in his own hell- Frederick Nietzsche