The best resource online for free camping can be found here
On my site I will only post places I have personally stayed and will add short reviews of my own experience.
I stayed overnight in Cabelas parking lot late Oct 2017, right off I-75 in OH. The folks at the store couldn't have been nicer and I felt very safe and my rig secure. They told me I could stay up to 3 nights. I only needed 1.
Only drawback was noise from thruway. Map below: