We camped at Mary E in 2016.
We did have a vehicle pass, though it really wasn't needed. But, we stored our instruments in the truck at night.
We got there Monday evening, and there was still plenty of spots available. We loved the spot we chose. ALL the way back, and to the right.
Couple of suggestions:
Mary E is very close to a river, so it get's much colder at night, compared to in town. We were one of the only campsites with a propane heater, and were very popular with the campers in the back 40.
There is absolutely zero lighting walking back to your campsite at night. So, alcohol + grabbing the last bus back to Mary E after midnight and a GreenSky Nightgrass show was definitely aided with a flashlight of some kind.
Did I mention it's probably 10 degrees colder at Mary E at night?
(Pack clothing accordingly)
You are probably looking at a 90 minute round trip, if you attempt to leave the festival during the day/early evening, for Mary E., and back again. We started each day in shorts/T's, but packed jeans and extra layers in our back packs, and layered up after dark at the venue. So, we didn't ever return back to camp, until the day was over.