Thousand Islands and then to Maine

The only times I found any difficulty was on weekends, and even then I had no problem if I called ahead during the morning for a site that day. That said, I usually stop by 2:00 - 3:00 for the night. If you plan to drive until dark, that might be more of a problem since some of the campgrounds I stayed in during June filled by weekend evenings.

If you are willing to camp without hookups, Lamoine State Park in Maine isn't too far from Arcadia and does have hot showers, etc. I also enjoyed both Robert Moses State Park on the St Lawrence River and Southwick State Park on Lake Ontario in NY, both with electric only hookups, and hot showers, all part of a June & July trip to Nova Scotia.

I think it will depend on the area and the campground. Just call ahead to see if there's availability, and if it looks like things are booking up make a reservation. We've done last minute bookings at the Bar Harbor KOA with no issue but we've also tried to book in advance with no luck. It's getting close to back-to-school time so sites are starting to open up.

We never make reservations either and had no problem on the Maine coast two weeks ago.

I see from a previous post you have a 5th wheel. I don't think you'll have a problem in Maine. Not sure about upstate NY.