Did my 3 yrs there, also, but must point out that I left in '98, so some of this may be OBE.
We lived in Swansea, a bedroom town north of Belleville, and south of Fairview Heights (but you'd never know you left one for the next given the amount of development.) FH is the shopping area: mall, big-box stores, etc. Very few people live in FH. Belleville is mostly older homes, a "cute" downtown (so sayeth She), and long established local businesses. Plenty of folks lived in Swansea, it's more convienient to the shopping of FH and I-64 (the route into STL).
That said, Birdstrike is right, lots of folk live in O'Fallon, good schools, houses a little pricier than Swansea/Belleville. Still good access to I-64 and closer to the base than Swansea.
Shiloh is very close to the base (we'd go there for lunch sometimes). Lots of strip-the-land developments, and 1 restaraunt. Don't know about schools. Mascoutah is farther east of the base, much farther to I-64, STL, etc. It's a "country" town. If you must have large acreage, and can afford it, then maybe it's for you. Personally I'm a big-city guy, so like Birdstrike, I avoided it.
As to schools, there is a historically large Catholic community there, with many Catholic schools. So many so that they had room for my kids, even tho we're not Catholic. Swansea public schools had ranked well, but we went private after disagreements with the administration. This is not an ad or endorsement for or against Catholic schools or the Church, just a statement on the availability of said schools in the Belleville area.
Good luck to you.