If there are no issues with site fee's ie. winter storage agreements, outatanding payments for last season, or issues with damaging the site grass, then take a drive to your local police station and ask their assistance to allow you to remove your caravan.
If there are no issues of payment the site manager will have to give the police reason why the caravan cannot be removed by yourself. Do that to-day.
Its a shame you cannot mention the site by name.
Tell us what the police say..