I plan to erect a big plate steel the other side of the wall to hold the cabinet.
There’s no reason why you should not keep firearms in a static caravan but as you recognise the security requirements differ from those for a house.
Your caravan should be fixed to the site or at the very least have no wheels or towing hitch so as to prevent it being easily towed away.
Nor should it be possible for the ungodly to gain access to the underside of the van.
Normally the gun cabinet must be bolted to the chassis of the van.
These are the main considerations but your FEO will give further advice, if need be, when he visits.
I suggest you discuss security with the police before going ahead and making definitive arrangements.