Caravans have rigid insulation within the walls which provide stability to the caravan structure.
The older caravans have 20mm rigid insulation within the walls which is not easy to get nowadays (i found some at FGF LTD 01733341717)
The roof void and front a posts seem to have rockwool (any rockwool type insulation will do)
It does not need to be fire retardant (it would be good if it were) but remember the rest of your caravan has polystyrene insulation throughout the walls so if it's on fire your standing back.
Keith Dixon
October 2008