You pretty much hit the nail on the head with the demographic point. Very White, rural/small-town, less college+ educated than average, quite Catholic. This region exemplifies the kind of working-class economically center-left, socially center-right voters that Trump activated.
Yeah, I checked out the counties in and bordering Illinois and the main thing that stuck out to me was that they were very white at >=90% and had a declining population (generally they were 20-30k).
Apparently this region is known as the Driftless Area
https://en.wikipedia.org/wiki/Driftless_Area
