Campground near Decatur or Springfiled Illinois

There is the fairgrounds in Springfield. It's 5 miles from I-72. The fairgrounds can be full other than during the state fair. There are horse shows and other events all thru out the year and many of those fill the campgrounds. Some of the fairgrounds campgrounds is on grass with some shade and some of it is in the asphalt paved parking lot with no shade. Quite a few of those in the parking lot take quite a bit of blocking to get level.
There's also Carpenter Park a city campground north of the fairgrounds in Springfield. It's 5 miles from I-72. It's grass with limited shade.
With both the fairgrounds and Carpenter's Park they're close to the city sewer plant. Most of the time there's no smell but if the winds are coming out of the right (wrong) direction then you do get some odor.
Double J south of Springfield is the quickest and easiest to get in and out if just passing thru.

We also used the state fair campground in Springfield for an overnight stop. It was as dbbls described it. Worked well for us.

Double J is where we stayed. Long level pull thrus with shade. Nice managers.

Friends Creek is about 15 miles east of Decatur and about 3 miles off I-72.
Just east of Springfield is a city park at Riverton, IL on the Sangamon River. It's only about 1/4 from I-72 but is about 2 road miles to get to it on US 36. It's not well marked to find. If interested in that one I can get you the GPS coords.
Around Springfield, 7 miles south of I-72 on I-55 is the Double J. Easy to find, easy access, quick on an off the interstate. Fairly decent campground.
Those are your choices as far as close campgrounds. Personally, I'd do Double J.

I have stayed at the fair grounds in Springfield. All concrete with full hook ups. Just be sure the fair is not going on when you are there.

East of Decatur on I-72 about 15 miles is a little place called Friends Creek. Its a little off the interstate, maybe 2 or 3 miles. Water and electric (30 amps) dump station. Nice little place ,would be a nice place to overnite. Clean showers and restrooms too.