I used to live about 5 miles from Cunningham falls. I can tell you that Cunningham falls and Greenbrier - which are the 2 closest CG's - won't fit your rig very well. These are older, tight State parks, in the mountains and even if they have a site to fit it, it's white knuckle getting in and out.
Probably the closest CG's are in the Gettysburg area. It's about a 20 min drive from Thurmont and there are a few that are full hookup.
Outside of that, you have Rambling pines but it's a little off the beaten path, Yogi in Williamsport and KOA in Harpers ferry. They're all about 30 - 35 mins from Thurmont.
On Edit; I would book sooner than later. The dates you are talking about is "leafer" heaven. People come from all over to Thurmont for the mountain leaves and Fall fest in Thurmont. So many people that cars get parked lined up on the highway and off ramps. It's nuts.
