I just got off the phone with National Motorsports, they provide insurance for off-road parks. For this size of of park (which is considered to be a small park to them) insurance would be between $8k to $10k annually. This only covers the trails. So you have to go through a local insurance company to get liability to cover the pavilions and the cabins. I'm guessing this is why we see a lot of parks that has plenty of space for RV or primitive camping and not a lot of cabins.
They will also provide the release waiver for the park.
She made a comment that a lot of the smaller parks don't carry any insurance and rely on waivers alone.
I wouldn't go this route but, TN is one of those states that highly enforces waivers and they also have laws in place that provides protection for OHV areas, public and privately owned, against prosecution.