Although many sites are open all year round some have council licencing restrictions which limit the length of stay, for example max 3 months. So you need to check that, others have restrictions on residential vans.
It can be a question of chatting to the owners, a CL we often use has a two berth caravan parked in a remote corner and someone's been living there for at least 3 years.
What you gain is freedom from the tyranny of the static site owners. If they turn awkward you can hitch up and leave.
Final thought, you need to let any insurers know your living in it, our policy makes it clear it covers holiday use, not permanent living.