I caught a ride back from Hawaii on a MATS C-141 years ago. It's
a cargo plane, and has exactly two porthole windows (one under
each wing, presumably to check to see that the engines haven't
fallen off) in the fuselage - exclusive of the cockpit. When
installed, all passenger seats are in the middle of the cargo
deck and face the rear. The toilets are on a pallet - rolled in
and fastened down. A glorified porta-potty.

Talk about "taking the tube".... Definitely not a fun way to
travel. However, the price was right.