Glen Burnie is pretty far from Thurmont. Cunningham Falls State Park Is right in Thurmont. They have sites that would fit Your DP but it would be tight getting in. However if I can fit my 39' fiver you could fit your 39'DP.
If state parks are not your thing, consider the Gettysburg CG's.
Granite Hill, Drummer Boy, Artillery Ridge, Gettysburg KOA, Round Top, are all closer to Thurmont than Glen Burnie.
If you want to be closer to the Balto. Metro area Ramblin Pines is on the west side of Baltimore in Woodbine, MD not really close to Thurmont but still a good 45 minutes closer than Glen Burnie.
