I used to live near Hagerstown, I would have just let you park at my house. But we moved South a few years ago. An overnight on the Saturday on July 4th weekend is going to be impossible at any CG or SP. All have a 2 or 3 night minimum and will be booked solid. If you are just looking to catch some rest I would try the Wal Mart. Keystone Rv in Greencastle is only 5 minutes on I-81 North of RT 70/81 interchange. They do have a small CG next to the shop. They might have a spot. There is also the Valley Mall which is right at the 81/70 interchange. While I've never seen campers there, you could call the mall office and see if it would be a challenge to park there. I've parked there for a few hours while we had lunch on our way home from visiting family. There is a very small, little known CG called McMahon's Mill not far from Yogi. It's run down and a bit overgrown, but they do have water and elec (although you pay extra to run the AC). I stayed there once and they had a few low limbs that made me wince but no damage. No one stays there so there's a chance they have a spot. They don't advertise and I think they just get what occasional camper they get. Good luck.
But if you want to try your luck on CG's - HGR has - Yogi, easiest to get in and out off 81 but expensive and 3 night min. battlefield KOA (not sure why it's called that, it's nowhere near the battlefield), Greenbrier state park, nice but not too big rig friendly - Saturday's are their busiest days with people coming to the lake for the day. Cunningham falls state park - little out of your way but not too far. And Mcmahon's mill, which is all but out of business but may have a spot for you. That's it for Hagerstown.
On Edit. did a little poking around and it looks like Mr Mcmahon died a few years ago and the Mcmahon's Mill CG seems like it's totally abandoned. Shame, in it's day it was a pretty cool place and had a restaurant and museum on site.
