A bit off topic but is this because caravans are very easy to steal? If so do they also sometimes get stolen if you are just in a campsite and out for the day?
As Other Clive says caravans are easily stolen so good locks are essential. The C&CC did publish details of where they are stolen most a few years back and the answer was when they are stored. The highest rate was from un protected storage sites and then from homes with CASSOA sites well down the list.
There were a number from the likes of services but they are low but should not be ignored. A lot of people do just leave a caravan unprotected and go for a meal or whatever and opportunists have hitched them up and been gone before the owners get back. It is rare as the thieves do not what may be on offer and normally are looking for something specific, so storage is more popular with them.
Sites in season are less common simply because people tend to recognise the people in nearby caravans making theft risky, sites tend to have barriers making t difficult to get in and out and cameras that can pick hem up. In winter though many ae simply stored there and could be vulnerable.