I just hiked the AT through there last year but it was late April when i passed through.
Mile 0 Harpers Ferry (270' elev)
Mile 6.4 Ed Garvey shelter (890' elev) has room for 10 hikers plus tent sites. The spring is 1/2 mile downhill.
Mile 10.5 is the Crampton Gap shelter with intermittent spring (1,000 elev)
Mile 15.5 are the Rocky Run shelters (970' elev) Spring at shelter
Mile 17.3 is the Dahlgren Back Pack campground (990' elev) water, showers, picnic pavillion HWY 40A close by.
I only needed a permit for the Smokeys and Shenandoah's.
You said an overnight trip so i limited the info to 17 miles northbound.
I took the data from The Thru Hikers Handbook by Bob McCaw.
If you need more let me know.