Decatur Illinois Camping

The immediate area around Decatur is sort of a dry hole for campgrounds. Here are some of the closer ones.
Friends Creek Conservation Area is about 15 miles NE of Decatur. We've been thru there several times but have never camped there. It looks nice enough. Nothing fancy.
Near Cerro Gordo is Tom and Carols RV Park. Cerro Gordo is about 10 miles east of Decatur. Can't think I've ever driven by this place and paid any attention to it.
About 30 miles north of Decatur outside Clinton is Weldon Springs SP. We haven't camped there in a few years. The state of IL let it get really run down and we haven't been back to see if upkeep has been done. It was at one time and could be a nice park but it got so bad it was rated as one of IL's worst maintained SPs.
South of DeWitt which is about 35 miles north of Decatur is the Clinton Lake Recreation area. This one is very well maintained, nice spacing between sites, all level sites. Some are FHU.

Give this site a try. Doesn't show all the private campgrounds, but is pretty complete:

USCampgrounds

RV Park Reviews says there are 25 of them within 50 miles. Their web site should help you out.