The Rising Sun Campground (late May–mid-Sept., $20) isn’t particularly appealing in itself—83 shrubby, often hot sites—but it’s just across the road from St. Mary Lake [1]; there’s a hiking trail heading up Rose Creek to Otokomi Lake, and there are pay showers in the nearby cabin complex.
Just inside the park boundary, the rather drab St. Mary Campground (800/365-2267, www.recreation.gov [2], late May–late Sept., $23; Apr.–late May and late Sept.–Oct., primitive camping $10; Nov.–Mar., no fee) is about twice as large as Rising Sun but is not so well positioned for boaters and hikers. However, it is possible to reserve a site in advance.
There’s a KOA (406/732-4122, www.goglacier.com [3], May–Sept., $28 tent, $35–48 RV) in St. Mary with tent sites tucked among riverside trees.
Links:
[1] http://www.moon.com/destinations/montana/glacier-national-park/logan-pass-st-mary/sights/st-mary-lake
[2] http://www.recreation.gov
[3] http://www.goglacier.com