I have camped at Apple River Canyon several times over Memorial Day weekend and loved it.  The state park offers a small camp area, hiking, and fishing.  It isn't too far from Galena if you want to head into town and wander around.  Some camp sites are reservable, some are walk up only.  It is about 50/50 with people camping in a tent vs bringing in an RV.  https://www.dnr.illinois.gov/Parks/Pages/AppleRiverCanyon.aspx