I have spent the past week on the construction of the camper base and roof frame. 

The camper frame sits on top of the frame on the base of the van and the steels match the postion of those on the base. The steels are mainly 40mm x 40mm square mild steel tube with 3mm wall thickness. The back front and sides of the camper base are 80mm x 40mm 3mm rectangular tube. There is quite a lot of guess work in this but I think that I have struck a reasonable balance between strength and weight. I have had to think of the twisting that might occur as the camper unit is demounted.

I have experimented with creating a laminated floor base on 4mm Ply, 30mm XPS styrofoam and 12mm ply. I have stuck these three layers together with an expanding foam glue. This has produced a (very) surprisingly light and strong composite board which will support my weight without flexing between rafters at 800mm centres. On the basis of this I plan to construct both the floor and walls on this principle. XPS is expensive so I have done some further experimenting with PIR (Kingspan) board. I was not sure how the glue would work with the foil face on these boards. It turns out that the glue works fine so I have settled for this. The PIR board is 40mm thick. This is thicker than the average caravan wall so limits the choise of windows. I have opted for Seitz S4 windows which can cope with this wall tickness. 

On of the reasons for making the camper demountable is to enable use of the vehicle for other uses when not on holiday. Such as.....

I used the base as a template for making the roof frame. Once complete this was moved the my scaffolding workspace. I have built this as a stand on which I will build the camper. This keeps the van free for other work and means that I can work from a platform whch is level so the spirit level can be used. Once the roof frame was welded on the truck it was transferred to the scaffolding, turned over and the welds completed to the underside.