We used 50mm polystyrene encased in the steel frame by 12mm plywood boards and expanding fire foam. This is a pretty straightforward process: cut polystyrene to size, cut plywood to size, slide polystyrene into frame, spray expanding foam on board, secure board to the steel frame using metal screws, repeat.
This is also the stage at which we put in 3 windows and cut out the crawl-through to the cab. We put one window in the sitting area, one in the kitchen, and one over the bed. These required building 63x38mm timber frames custom for each window which were then epoxied to the aluminium panels and screwed into the box frame. We used dome screws to secure the edge of the aluminium panels to the window frame from the outside. The crawl-through is also made up of a timber frame screwed into the steel.