Chicago to Upper state New York and beyond questions....

I would take US 20 through Indiana and stop at the Amish places. Fall will be a pretty time there. I would join the Interstate system in Ohio

Unless you are really into toll roads I would suggest a different route. I have my own I-Pass so know about them. Once you et into Michigan take I-94 to Benton Harbor area and then I-196 toward Grand Rapids. When near Grand Rapids take M6 toward Lansing and I-96. Lansing has a few interesting th9ings to do also. Take I-69 from Lansing to Port Huron, MI./Sarnia, Ontario and the Blue Water Bridge. It is an easy crossing and you will be on 402 to the 401 in London, Ontario and then to the 403 that will take you to Niagara Falls. As you are going through Flint, MI. you can take a short trip up to Frankenmuth, MI. for the famous chicken dinners and Bronner's Christmas store. Those roads in MI. are not the ones you hear about that are so bad. I just did all of those roads in the past 6 weeks, including the Canadian ones, and they are all good. Detroit area has the bad roads. The heavy traffic on I-94 makes it also not as good but you would only be on I-94 for 28 miles and most of that has been redone already. Since the best view of the falls is from the Canadian side I would also recommend you stay on that side for a day or 2 and then move to the US side. While on the Canadian side take a side trip to Niagara on the Lake. The Shaw festival should be going on while you are there. It is a great little town for toads but not RVs. You could take a trip to downtown Toronto on the train from Niagara and see the sights there also. The public transportation is great in Toronto. There is a great small bakery in Strathroy, (off 402) that has great butter tarts.
When I need Canadian money I use an ATM and get the exchange rate immediately. 2 weeks ago it cost me $243. US for $300. Canadian. 3 weeks before that it wasn't quite that good.
If you fill the MH & toad in Port Huron you can make it through Canada without buying any of their expensive gas.
If you are into beers keep in mind that Grand Rapids is Beer City with all the micro breweries there. Downtown has the Gerald Ford Museum and a lot of other interesting places. I just had 7 children to the Grand Rapids Public Museum that is across the street from the Ford museum.
Enjoy your trip

You didn't say where in IL. We're in western Chicago burbs and go back to Ithaca NY to see grandkids every year. Take I 90/80 to 480 at Cleveland to I271 back to I 90 in OH. Driving at 62mph, we stop at Erie KOA after about 8 hours drive time which includes fuel stops, food, etc.

Wife has had it at 8 hours on the road, no longer likes those 500+ miles per day

That puts us about an hour +/- from Buffalo and Niagara Falls.

Be sure to have or get an I-Pas, saves time and $$$$$.

Coming back home, drop down to PA turnpike to get to Gettysburg are and western Maryland for Antietam battlefield.

If you need an overnight stop in OH, the turnpike has RV spots at MM75 & 238. $20 a night.

Your trip to Niagara is pretty simple. Take I-90 east. Chicago is about 550 miles from WNY so it could easily be done in one day. All of the trip is expressway. If you do not have one, get an EZ Pass before you leave. That will save you some dollars and a lot of time.

Most will stay at the KOA on Grand Island. This campground gives you easy access both to Buffalo and Niagara Falls. Make sure you bring your passports because the best place to see the Falls is from the Canadian side and there is more to do on that side.

Things to do in the area range from the historic, Teddy Roosevelt swearing in site at the Wilcox Mansion, the arts, Albright-Knox Art Gallery, the architectural, Downtown Buffalo and the Frank Lloyd Wright homes, the old car, the Pierce-Arrow Museum, the floral, the Ontario School of Horticulture, and even the exciting in the jet boat ride into the Niagara Whirlpool.

I hope that helps at least a small bit.