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I would guess the Detroit crossing would be the fastest and cheapest considering toll roads. There is a Sams Club for overnighting at exit 37 SBD I-75. That's the first-easiests and safest one to get to I can think of.

As far as "must see", that depends on what kind of things you like ie.: museams, nature, opera, plays, ball games, hiking, etc., etc., etc.

We use the RVPARKY app when on the road. It gives you locations of campgrounds, state parks, walmart, cabelas, loves travel centers, cracker barrels all along your route.

On your way back, if you've never been to the Biltmore Estate I think it's worth a looksee. There is always something to do in Knoxville and Pigeon Forge/Gatlinburg area. There is Pilot Mountain in Mt. Airey NC....AKA. Mayberry of the Andy Griffith show.

With the internet today it isn't that difficult to find interesting places to stop.

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.

Why would you cross at Sarnia? Just for the RV park at the bridge? That adds at least an hour or two to the trip.

I would not argue the point. After all, I do not live in that part of the country. BUT I just checked with Google Maps and the difference was 6 minutes longer by Port Huron.

My answer to "why" is that I cruised right on through Detroit on the Interstate and trusted that it was less crowded and probably quicker to go on up to Port Huron to cross over. I may have been mistaken, but that is my answer to "why".