If you're headed downtown, PWK is probably your best option distance wise. Not sure about the fees there for something RV sized. I could tell you all day long how much they would charge if you were in a corporate jet but not sure about GA. I have seen plenty of single engine stuff there.
Don't overlook Gary, Indiana (GYY). Gary is also a great choice to get to downtown Chicago. If you're coming from the south or east, it's way easier IFR wise to get into and out of GYY than PWK. The Gary Jet Center is a brand new FBO.
If you're coming up from Alabama, you will get the tour of the far west side of the Chicago Class B as an IFR arrival into PWK. GYY is probably a direct to the airport arrival.