When buying a caravan, and you're told it is "fully insulated", it can mean a whole range of things:
Most vans are built something like this – a timber frame with pieces of styrofoam glued into spare space.
Some vans use a fibreglass wool insulation (like used in a house) – however, this tends to settle after a few kms on the road and the fibreglass wool compacts down to the bottom, leaving space above it. Even if it is glued on, only a thin layer of the fibreglass wool remains attached to the original location.
Composite panel construction (smooth sided vans) that have a bonded core don't suffer any of these limitations and provide superior insulative properties.