Rigid airships are not full of helium (or hydrogen). The outer skin is for aerodynamics, not to seal the gas. Inside, they have a series of gas bags affixed to

Rigid airships are not full of helium (or hydrogen). The outer skin is for aerodynamics, not to seal the gas. Inside, they have a series of gas bags affixed to the rigid structure that can expand or contract according to the pressure at altitude. At full altitude, those gas bags would expand to their full size and take up most of the volume in the airship, but there was still a lot of space around them (and inside the trusses) for storage, maintenance access, walkways, and so on.