We have made two RV trips to Ontario, staying at 50 Point Conservation Area near Hamilton, south of Toronto. We have crossed both at Sarnia/Port Huron and at Niagara Falls. We have intentionally avoided the crossing at Detroit. That would still be my plan to cross at Sarnia over to Port Huron. There is a nice RV park right at the border in Port Huron where we have stayed.
You could swing east on I-75 to the Lexington, KY area. There is a great RV park at the Kentucky Horse Park, just off the freeway, north of Lexington. You could take the Bluegrass Parkway back to I-65 at Elizabethtown, KY and continue on to Nashville.