It starts at Adams and Lake Shore Drive (or Jackson in the other direction). This is Grant Park, on the east side of the Loop. Apart from big city traffic there is nothing bad or dangerous about this. I don't see anything wrong with the drive west on Adams to Ogden either. Ogden is a major, multilane road heading SW to the Chicago suburbs. Apart from lots of lights it should be a breeze, even in the largest RV.
http://www.historic66.com/illinois/det-il1.htmlObviously if your preference is quaint small towns, then neither end point is your cup of tea. You'd also want to skip St Louis, Tulsa, Oklahoma City, etc.