We stayed in the McCormick Place truck marshaling yard for a couple nights in 2017. We're planning another visit this summer. We don't tow a car, so we walked and used Uber/Lyft to get around. The location is very convenient for exploring all that downtown Chicago has to offer.
http://www.mccormickplace.com/attendees/parking.php
I learned about it here:
https://www.gonewiththewynns.com/downtown-chicago-rv-camping
It is not an RV park. There are no electric/water/sewer hookups. It is dry camping in a parking lot with semi trucks, buses, and other RVs.