Back to the OP's question concerning security. Your insurance company will stipulate what the minimum security measures you need, and when they must be employed. If you fail to follow their direction, the policy won't pay out in the event of a loss. There is no "standard" for what insurers require, so you have to follow your specific insurers guidance.
Similarly storage sites will have their own terms and conditions about immobilisation and each site may be different, so check with your site. And as sites do not insure your property, your insured t&C's trump the sites, and if they don't match then either change insurance or the site until both sets of t&c's do work together.
As for what you want from security, you want it to deter the criminal fraternity from going after your caravan. As others have pointed out, if they really want your caravan, they will have the means of physically taking it, so you need to make it less desirable than someone else's.
Thrives don't like things that will render themselves more likely to detection, so look for ways to slow them down, make it obvious the caravan does have security measures, things that will allert the owner/authorities if the caravan is compromised. Trackers to help locate the caravan if it is stolen. Remove valuables (both fiscal and emotional). Use a storage site that has better than average security, such as keeping the caravan out of the public gaze.
Keep the caravan and it's contents insured, and of course locked.