thank you I will let you all know my findings when I get back from the retreat
Michaux State Forest covers Adams, Franklin, and Cumberland Counties. Public safety radio coverage is much better than cellphone coverage throughout most of the forest, including Birch Run Road. I only know of one cabin on Birch Run Road and it's on the northern end of the road, but as a general rule, the closer to Big Flat you go, the higher the elevation, and the better the coverage. I have used a G5 in that area for several years and have had no issues with reception, monitoring mostly Adams County, but also some Cumberland and Franklin. There is an Adams County tower on Big Flat and, as someone else mentioned, you can look across the valley and see the Franklin County Clark's knob site. Franklin County can be received without issue that high on the mountain well past Pine Grove Furnace State Park. Once you get down off of the east side of the mountain, towards SR 233, you'll lose coverage, especially north of Shippensburg Road.
The Cumberland County line is only about 2,000 feet west of Birch Run Road, on Shippensburg Road, and there are UHF simulcasts available of their fire/EMS OpenSky talkgroups in the Shippensburg area that you can easily receive from the Big Flat area. Frequency information is available in the RR database.
Also, just so you are aware, Birch Run Road is completely located within Adams County and the only place that it gets anywhere close to Franklin County is down near the reservoir on the south end of Birch Run Road, near Milesburn Road. You can easily monitor the Franklin system anywhere along Birch Run Road, but you won't hear much activity related to the area you're staying if you are on the northern section of the road. You'll hear a lot more local activity, to the extent there is any in that area, from Adams and Cumberland.
Lastly, with regard to cellphone coverage, most of that area does have very poor cellphone coverage. I have mapped coverage of all four major carriers throughout most of Michaux State Forest using mostly Cellmapper, but also a few other cell coverage mapping apps, and in that area, AT&T, seems to have the best coverage. They have a tower located at 1700 Ridge Road, which is the only cell tower located anywhere near where you'll be staying, although in spite of that, there are still plenty of places along Birch Run Road where even AT&T drops coverage. If you decide to go to the reservoir, look west from the boat launch and you'll see the AT&T cell site. There is a Franklin County repeater co-located on that tower as well, which is why the coverage of that system is so good in that area.