Chicago - Milwaukee (suburbs) - Madison
That's where you get the GPS file. Make sure to shut off automatic routing or it's useless. The trails in Kenosha and Racine are worth the entire first leg. There is also fresh pavement going on in the old crushed stone sections.
I doubt you'll have trouble getting a campsite, but check first at Cliffside.
I really don't have much to say because I worked out all the kinks in this route. Just have some nice touring tires, like 28 - 32 if you're riding 700C. I think most 26" tires will do. You will be on a few crushed stone trails. I chose trails because I didn't want to deal with cars - less cars equals fewer close calls, etc.
Don't bother with bringing a stove and food as there are plenty of places to eat on the trails. Do bring a few energy bars, water (water is a big one), and maybe some fruit. There's a great taco place in Waukesha but I don't remember exactly where in town it is.
..and this time of year bring plenty of bug repellant. I like this route in early spring when the 'skeeters haven't quite recovered from winter.