Don't know about Saturn rockets, but hydrazine and nitrogen tetroxide tanks on Russian spacecraft usually contain what amounts to a nitrogen-filled balloon under pressure. As fuel gets used up, the balloon expands and keeps forcing the fuel against the intake valve; there is no free gas in the tank. I don't think this works with cryogenic fuels though -- no elastic substance retains its elasticity in contact with liquid hydrogen.

__________________
Fiction has to be plausible. Reality is under no such constraint.