For Illinois download the FastPass app for your iPhone, (I'm fairly sure they'll have it for Android too).
It requires no transponder because it uses the GPS in the phone to know that you are passing the toll area - it usually alerts me about three seconds before I pass under the contraption that reads the transponders so it's very accurate.
I regularly travel from Detroit to Milwaukee with and without the trailer and I just flip plate and axle numbers just before paying. We generally go round I-294 because it is generally the one that has the least traffic issues. Without the trailer the total cost is around $5-7 and with the trailer it goes up to about $20.
Do _not_ forget to pay the tolls upon completion of your journey. I did so once. The send you no warnings or reminders and six months later you get a bill for $194... :S