A lightweight modular construction for use as a slide-out structure in a recreational vehicle is provided. The slide-out structure comprises a floor, a structural wall, and a roof. The floor, the roof, and the wall structure may include a plurality of reinforced foam panels, connected together in a modular fashion. A first and a second elongated bracket interconnect the floor and the structural wall by an interlocking dovetail connection. A header bracket interconnects the structural wall and the roof.