The muties do not live near the outer hull; Heinlein clearly states that mutie country occupies the decks with low/zero weight. There is no complex tech providing artifical gravity, nor is weight equalised from hull to centre by differentially rotating decks. This is clear from the description of the gradual increase in weight experienced by Hugh and his friends as they return from their expedition to the upper decks. The only answer which fits is that the Vanguard is a vast cylinder containing concentric layered decks, with weight produced by rotation. The rotation is explicitly mentioned at the climax
when Heinlein describes the guide rails for launching the lifeboat; they are curved to offset the Vanguard's spin.
