We'll be there in November. Online, lots of sites say "Not available for reservation". Does that mean they're already booked, or are they not reservable this time of year?

Means site is already reserved. But they have cancellations every day and start offering them around 9:00 am each day!

When I was there, online reservations took priority over everything. They must already be booked or the sites you're looking at are closed

Have to plan and make reservations months ahead...we passed through and was able to get one night with NO hookups for $35

Already booked. :(

It means site is already booked.