I'd stick with I-80 to I-39. You can't avoid the ~20 miles between the I-65 junction and the I-294 split, but you do minimize your time dealing with the general craziness that defines the southern end of the Chicago metro area...
Taking 80-39 is 30 miles longer, but you won't have the tolls that you will have on 294 and 88. You also escape the metro area much, much sooner.
Cash tolls for 284-88 are ~$14.40 for the RV ($9.60 for a car).
You still have to pay a toll for 10 miles of driving along 80 to the 294 split, but that's just $1.10 for a car or $1.70 for an RV.