A camping forum for VW bus and camper owners.
Located just off Montana Highway 1 to the West of Georgetown Lake in the Pintlers. Probably a great place to try when the campgrounds around the lake are full up.
Some of the 16 sites are huge and almost any could accommodate up to three VWs and all their people. The highway is up above but isn't horribly busy and at night is dead for the most part anyway. A local favorite for hunters, so it'll be busier during hunting season, but the sites are dispersed far enough from one another that I can't imagine you'd notice any but the noisiest neighbors.
No water and vault toilets, but no fees either. If you have some good filtration equipment you could just drink out of the stream that runs through the entire campground and isn't far from any given site; indeed, it's right behind most of them.
Official Site (includes map)