Static Caravan Insulation

[Ed21]

I've been asked to help replace about 4m2 of an exposed outside caravan wall. 
Inside is 6mm faced ply glued onto 2X1 studding and the thinnest of rockwool insulation
that no way covers the whole area. 
On the outside it's got 9mm ply with a thick resin type stipple. 
None of this has any vapor barrier.
 
It's easy enough to cut out and patch with new plywood on the outside, but was thinking
at the very least to add some kingspan. 

Gap is 50mm, so to keep it simple, was thinking of 40mm board to allow a small 10mm gap. 
Question is would the gap be better on the outside edge or the inside one?.

 

[PeterW]

Can you not find the studs and mark them, then put a 30mm sheet of Kingspan, 9mm OSB and then screw through into the existing siding and studding ..? Then a coat of garage roof type GRP..? Cover the whole side and you’ve insulated the whole thing and no messing around ..?

[Ed21]

As it's just a repair got no option but to match the existing, adding depth or a different surface
they won't like (not my call TBH). 

The outside ply has to be removed anyway as it's delaminating in places, so thought why not add
some way better insulation. 

Options are to fully fill the void.
Partially fill the void with a gap before the inside ply or partially fill the void with a gap before the outside ply.
Doesn't have any barrier at the moment, but if added could add one pinned to the outside of the battens only. 

I'm sure kingspan will be better than what's there now, but don't want to add if I end up making it worse.