There are plenty of hotel options near Race Headquarters, but if you are interested in camping, you have two main options.

Option #1- Rock Cut State Park  - https://www2.illinois.gov/dnr/Parks/Camp/Pages/RockCut.aspx This option will put you in the same park as race headquarters. This is an ideal option for 4-hour racers.

Option #2 - Blackhawk Valley Campground - http://www.blackhawkvalleycampground.com/ This option might be a good one for 10-hour racers since it is within minutes of the bike drop early in the morning. The only issue is that it is a 25 minute drive from race headquarters and the finish of the race. 

If you are a 10-hour racer looking to camp it might benefit you to stay at Blackhawk on Friday night and Rock Cut on Saturday after the race.