St. Mary's is close to a lot of decent to good "roadside" places, the Park Cafe in particular, but right across the road from that is Johnson's (which I did not find memorable), and up the road is Two Sisters, then in Babb the Firehorse (?), along with other places I've never eaten at. Frozen huckleberry daquiris (with whipped cream) at Many Glacier Lodge (and quicky pickups, like ice cream cones and maybe heat-up hot dogs).
And you can buy one of the Park Cafe t-shirts in the general store next door: "Pie for Strength."
http://www.parkcafe.us/index.php
Check out the astronomers at St. Mary Visitors Center! Huge telescopes, deep-sky objects - nebulas you've only read about you can see for yourself - and they hang around until the last visitor is sated.
The A loop in St Mary campground is mostly for tenters; it has grass 'n' trees between sites. Possible to get a teensy bit of privacy. Sometimes a bear cub runs through it. Last year I heard coyotes yapping.
So, when comes the question about a laundry?
Babb.
West Glacier KOA.
Showers: Many Glacier, Rising Sun - Two Dog place (behind general store; they will sell you a token), maybe one at Fishcamp CG?, West Glacier KOA.
A lot of things will sort themselves out once you get there.