Because the coating is made of aluminum and not in the wrongly called fiberglass (its real name is fiberglass reinforced plastic GRP):
Don’t be fooled. Although it is true that the GRP itself has a lower thermal transmittance than aluminum, the caravans are not built of a single material, being aluminum only the outer coating, while in the caravans of fiberglass reinforced plastic (GRP) its entire structure is made of this material.
When we talk about thermal insulation, we must take into consideration the set of materials that make up its outer structure and that provide the caravan with a certain thermal conductivity. It is, therefore, a sandwich structure, composed of materials, in the case of Caravancol, of a very high thermal insulation capacity such as wood and high density polystyrene with a thermal conductivity of 0.02 K(W/mK) as opposed to GRP which is 0.06 K(W/mK).