The Dock Campground

First of all, St. Marys is a fun little town and this campground / marina is within easy walking distance of downtown. This is one of the last places in America where trains (long coal trains usually) go right down main street. The Campground is also in easy drive to Marietta OH, where one of the very first European settlements west of the mountains on the Ohio was established. Lots of history and stuff to see there.
And the campground is beautiful. Not much shade but it's in a field right next to a creek and island that connects to the Ohio. There are hiking trails on the island that are interesting. The campground has excellent utilities, level sites with cement pads, full hookups and a bath house.
All that said - this isn't really a tourist campground. Most of the rigs in here are either locals of keep their trailers at The Dock all summer or they are tradespeople working in the area. The side is managed by a family that lives a few blocks away and aren't used to dealing with tourists on short stays. They prioritize reservations for longer term customers.
I've learned the best way to contact them is via Facebook Messenger from The Dock's Facebook page.
There are quite a few campgrounds just like this one all along this stretch of the Ohio on both sides and they are all hard to make reservations for. Have perseverance and you'll get a beautiful and interesting place to camp.