There are several theorised ideas as to why some storage sites might not allow the caravans wheels to be removed or disabled. There is no unified standard across the industry so what might be true for one site might not be true for all.
The simplest solution is to ask your storage site for their take on the issue. If they don't allow stands or immobilisation, ask them why?
Also check with your own caravan insurance as to their security requirements, as again not all insurers have the same requirements.