We came across this one day during a day hike in the Marengo Ridge Forest preserve in McHenry county. It's about 90 mins NW of Chicago. The trails are both wooded and hilly, but there's also a large prairie nature walk. 
The campground has many secluded walk-in tent sites and a variety of sites for other types of campers. Many with elec and some are ADA accessible. The county website doesn't have photos, but does describe the sites well (" suitable for truck camper" "up to 40' trailer") Maximum people per site is ten, so good for family gatherings. Sites are widely spaced, and even the most open ones feel private. The campground is heavily wooded and there is a nearby pond.
 We camped here to spend time at the Railroad museum, 8 mi away, The weekend we were there, the museum was open till 9:30 PM, and the campground closes at 10, so it was perfect for an overnight stay. We had no problem reserving a space with just a few days notice (48 hrs min). There is a friendly host on site, and you can purchase wood in advance when you make your res and they will deliver it to your site. Good wood too, dry and easy to split.
Fire pits are low to the ground, except in ADA sites, so bring your own grill. The pumps have water with a strong iron taste, and the toilets are clean, but only outhouses. There is a dump facility.
Marengo and Union are both nice little towns with some good dining options; Niko's Supper club, Fire & Ice BBQ/Ice Cream and Clasen's a 120 yr old tavern. Cody's Orchard, 3 mi away, has Apple and strawberry picking, great fresh veggies, pies, donuts, cider and jams.