We did end up camping at Twin Mills for a short week in August. It is officially part of our Thousand Trails plan, but for tent-sites only. So we had to pay for the week at $198 for the week (or $33/night). Not bad for a full-hookup site.
During the week, we pretty well had our site and then next 4 beside of us open and free. When the weekend rolled around, it got busier and we ended up with neighbors.
We had a little bit of a hiccup the night that we got there. We arrived late- after 9 or 10pm because we came from another campground in Kentucky. It was close to a 400 mile day and it was a long one for us with traffic issues. When we pulled up to our site, our neighbor's 1/2-smoldering fire was right snug up with the edge of our site, their chairs partly in ours, and a car parked blocking us. We ended up looking at the campground map and found a pull-through area. We spent the night on that and went up to the campground office the next morning. They lent my wife a golf cart and she drove around until she picked out a new site (which ended up being very close to our original site).
My aunt, uncle, and 2 cousins came to visit us. We had a great week. The pool was nice. My only issue with the pool was that it's hours of closing at 8pm was a little early for us since I work full-time most days. It's hard to work all day, help and make with dinner, and still make it up to the pool before it closed.
Everyone we dealt with was friendly and nice.
I'd go back. It was a convenient location for visiting with my aunt and her family. We don't get to see them that often, so it was nice.