"But there is this cylindrical shaped opening that has metal around the inside and the back that looks like something should plug into it.
Don't worry about that opening ---- just attach your coax to the coax fitting, turn on the booster (green light on) and then attach your coax to the TV. On your TV select the correct source (Antenna or Air) and then search for available channels.
If you have access to cable at your campground, you'll need another coax to attach to the post at the CG and run to your trailer. Inside at the antenna fitting that your cax is attached to, turn off the booster and then select the correct source on your TV (Cable) and then search for available channels ----------