EZ pass should interoperate all the way through. Illinois is part of the consortium, even though they say "i" rather than "ez".
I've sometimes done marathon road trips (and enjoyed them), but what you're proposing sounds too ambitious for me in a motorhome. If nothing else, managing check-in and check-out times from the campgrounds that are somewhat reasonable and still getting eleven or twelve or so hours of driving (plus an hour or whatever for lunch and perhaps dinner) in seems quite difficult. I might be tempted to skip the campground and snooze at a Wal-Mart or similar. In particular, I'm not sure how you'll have enough time to see Niagara falls reasonably and cover the day's driving and get settled into the campsite at a somewhat reasonable hour. But that may just be me.
(Driving alone in a car, I find my driving ability and presumably safety takes a nosedive not too long after about ten hours on the road in a day. Obviously with another driver things are somewhat different, and people also differ in their fortitude.)