As I see it, accelerometers and GPS together are all that's needed. If the
vectors from all 3 accelerometers are in the right direction, and the
average GPS heading is not moving much- you're straight and level.

Ron-Using accelerometers in 3d space will soon become inaccurate due to
small errors in the accelerometers, math rounding errors and drift. How do
you propose to 'erect' such a device to keep the shiny side up?