One of my high school teachers was originally from between Marion and Mt. Vernon, and he said that he noticed that the boundary for Southern accents is around Champaign. So I would assume Champaign/Urbana is the furthest north you would get any sort of Southern IL culture.
One thing for sure, when I moved to the Chicago metro in 1975, I got seriously mocked for my accent. "Are you from Alabama?, Are you from Texas?" "No", I said, "I am from Illinois". "No one in Illinois talks like you do" is what I was told.
What I really wanted to say is if you actually drove somewhere the L doesn't stop, you would know Illinois is more than just Chicago. But after awhile I started running into other transplants, from Newton, Carbondale, Mt Vernon, McLeansboro, etc. and they all shared the same stories of mockery and adjustment.
Since I moved out of Illinois, the biggest issue I run into now is people who say "Illi-noise". I would say I correct about 2 people a month now on how to pronounce the state correctly.
Once I got stopped by 2 undercover CPD officers and they didn't even know where in the suburbs my residence was located. They just said "never heard of it"
On the flip side, I had to go to court in Jasper County (Newton) for a ticket once and the top #1 rule when you appear in a county court in southern Illinois is,
never say you are from Chicago! If you live in a suburb and the judge asks where your town is on your license, you just say "its in northern Illinois".