The Mount Adams [1] Ranger District has 17 developed campgrounds on the south side of Mt. Adams. Most are free, but there is an $17 fee at the larger campgrounds, including Moss Creek and Peterson Prairie. The historic Peterson Prairie Guard Station, west of Trout Lake [2], can be rented (Dec.–Apr., $59 d) and is popular with skiers, snowshoers, and snowmobilers.
Contact the Mount Adams Ranger Station in Trout Lake (509/395-3400) for reservations.
Other Forest Service campgrounds line Forest Road 23, the route that connects the town of Randle [3] with the northwest side of Mt. Adams. These are managed by the Cowlitz Valley Ranger District in Randle (360/497-1100) and range from free to $17 a night. Best among them is Takhalakh Lake Campground, which affords a gorgeous view of Mt. Adams.
Links:
[1] http://www.moon.com/destinations/washington/south-cascades/mount-adams
[2] http://www.moon.com/destinations/washington/south-cascades/mount-adams/trout-lake-and-glenwood
[3] http://www.moon.com/destinations/washington/south-cascades/white-pass-scenic-byway/morton-randle