I lived in Highland Park for 5 years and stopped at this travel plaza many times during the day and night. I've never heard of, or seen, anyone get towed for being there overnight, but I would read the posted signs carefully to see what they allow. Also, you can call 311 and ask... they are available 24/7/365.
If the previous ideas don't work I would call the IL state police since they patrol the interstate.
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