You can use:
RV Park Reviews to find campgrounds in the area(s) you're travelling to.
How are you getting to Niagara Falls? Are you going through OH, PA, and NY or will you just pop up in MI and go through Ontario?
If you stay in the US
Cedar Point may be a good stop for your teenager (and those young at heart).
The rest area's along the OH turnpike are a good place to overnight if needed--most of them have camper areas with electricity (although I think its $30 or something to stay overnight).
You could also make a stop at the
Rock n Roll Hall of Fame near Cleveland on your way by.
(Granted none of my suggestions are state parks, or "outdoors camping" but are decent places to visit anyway--if you're interested.)
I may have more but I have a meeting to get to LOL