We've left from southern Michigan in October before and didn't have much of an issue finding places to stop.
What resources are you using to find sites? Have you considered alternate routes?
That said, many northern campgrounds shut down laying off staff for the winter. If they have to keep say 3 staff at $15/hr for a month...that alone is probably around $7500-10000. At $30/n, that means they are going to have to generate 250-300 overnight visitors in that month, just to break even on staffing. I'm betting that's unlikely for a lot of parks.
Then add in potential damage from freezing (and complaints about the park that result). They also have to cover electricity (because people will be using space heaters. It's probably more hassle than it's worth.
Health department rules may require a functional bath house. Many aren't heated, so they can get in trouble being open without a bath house.
Many staff/owners are also snowbirds, so forcing them to stay into November/December may be a tough sell if they are staying in an RV and want to get south.
The further into the fall you push it, the harder it becomes to address these issues. If it is profitable, the parks will stay open (and some do) but for most, it's a money loser that comes with hassles.
Honestly, you best bet is to have the RV prepared for the conditions and get south fairly quickly if you are going to delay the departure late into the fall/winter.
