If a National Forest campground or other facility has a dump station, it'll probably have a modest fee attached. Lake Powhatan CG & Rec Area near Asheville NC used to charge $5 to dump, but they went up to $10 a few years back. I dunno if it still applies, but that used to include their day use fee, so you could use the other facilities (lake, picnic area, etc.) and stay all day if you wanted.
Jim, "Mo' coffee!"