Building my own truck camper

I am planning on sandwiching the walls as suggested. I just looked into the XPS foam and I will definitely try it as that could save a lot of weight. The outermost horizontal members of the overhead sections will most likely be supported by the 2x4 x12.5ft and they will stretch the whole length of the camper (just like your red ones on the bottom of the overhead are stretching a little to the back). I just noticed that it's not that easy to find the 2x3 x16ft. That will probably go for the roof long pieces as well. So the stronger ones would probably be at the same places yours are.

I have been looking at metal prices online, but the total would end up several times the amount of lumber. It's not that I don't want to use metal, I just don't have the resources to make it work. If someone has a suggestion on how to make it work financially I am open to the idea!