Hi there! Long time lurker here.

I've been dreaming about building an expedition camper shell for some time now. Recently I've been becoming increasingly interested in using SIP's.
The idea first came to me when thinking of how I could make an extended roof on a cargo van like this

But my daydreaming quickly evolved into something like a full rear shell like this

Would either of these projects be feasible? Can SIP's withstand the vibration? I'm assuming framing will be necessary too. Would I just be better off stick framing?

Your thoughts are greatly appreciated!

If you're talking about simply converting the inside of a cargo/utility van (like a Dodge Sprinter)?
You MIGHT. The generally, the width of SIPS and the fact that the interior is anything but plumb is going to cut down on usable space fairly dramatically.
Most of the attempts at conversion are more stick-frame and piecemeal of solid foam board combined with spray foam as they can then curve the walls and ceiling to conform to the structure of the van itself and maximize usable space.

I was actually asking about using it as exterior walls.

Think about building a truck camper from scratch on a flatbed pick up truck for example. The SIPS would provide the structure.

Or using it as the structure of a roof on a van that is not high-top. Picture cutting the roof of a ford E-series van and using sips the create an extended roof similar to the height of a sprinter.

If you're going to be driving the van anyplace, I'd say "no", as a construction like that probably wouldn't be street legal. as all that stuff glued/screwed to the outside would not be terribly aerodynamic and would constitute a road hazard if it blew off in transit.

I've seen people start from a flat bed or even a bare steel trailer.  I honestly don't know how mobile such homes are.  As I don't necessarily trust the builder's word.  Your best bet would probably some form of frame-reinforced metal SIP.

Instead, maybe look at a box truck?

http://tinyurl.com/BoxTruckExample

It'd provide more space, and the ability to frame and insulate more traditionally.