I suggest Indiana Beach, north of Monticello, Indiana. It is operated as a family resort camp and amusement park.
Another of this type is Camp Dearborn, near Holly, Michigan, though the emphasis there is more on cabin-like tent facilities, with the RV park as something off to the side. Also group camp and craft activities, rather than amusement park.
Attractions? I don't know what most people mean by that, usually everyone is looking for something different. Theme parks, theaters, natural wonders, outlet malls, water parks, museums, giant shopping malls? There is not much like any of this in Monticello, but Camp Dearborn is close enough to Detroit that you have 1 to 1 1/2 hour access to every attraction in the Detroit area, number one of which would be Greenfield Village and the Henry Ford Museum. But no Six Flags.
I like the Hannibal, Missouri area, with Mark Twain Cave being a nice campground. However, the park itself does not offer much in the way of activities, and Hannibal, as an attraction, does not have that much in the way of "keep me amused" for your age group.