I flew KC-135s from 1975 to 1988. When I started we had to wear parachutes and helmets for refueling and obviously no passengers. When I retired carring passengers to and from overseas places was normal. The plane has room for a bout 50 rear facing airline type seats or a round 20 people sitting in the luxurious side facing seats. The fuel is all carried in below deck tanks so the upper part is basically a hollow tube as can be seen in the picture. Baggage and freight share the space with any passengers. That's why the passenger capacity is so low.