I assume the site advertises this service. I have only ever stayed on one that does. In this case there was a long coaxial cable to put through the caravan window and connect direct to the TV in the usual way.
What you do is instead of connecting your short TV coaxial cable into the caravan booster box, you use a much longer coax and run it direct from the TV, through the caravan window and connect into the site box provided. What this does is bypasses your caravan aerial and connects up to a box that uses an aerial for the whole site which eliminates any setting up and immediately provides a better reception. The only thing you have to do then is just tune in the TV to the area signal which you would have to do anyway when you arrive at a campsite.
We have a 20 metre co-ax cable which connects to the bollard TV point and then to the caravan, either to the TV point on the outside of the caravan or if you don't have one I would run it through the nearest window to the TV and put the cable directly in the TV. If its a Caravan Club site the wardens often sell co-ax cable ready made for a reasonable cost.