It's draftier than she's used to and the hot water is, if anything, less reliable than in the Doctor's ship. Travelling through the Vortex in a red London bus is by turns thrilling and terrifying, with attacks from the strange things that live within its swirling chaos a regular interruption.
Iris' new body is certainly younger, and arguably more glamorous, but she's still at heart a temperamental plump, raffish, lady of a certain age. They both take a great deal of pleasure in rebelling, sometimes going places and doing things just to see if they can shock each other. Other times they do them just because they know the Doctor would disapprove.
For a while Sam gets the distinct impression that Iris is waiting for the other shoe to drop. For the rattling unreliability of the ship or the traumas of confined space and short tempers to drive Sam to demand that she be taken home or back to the Doctor. They have to get extremely drunk before they can have the necessary conversation. Stumbling home post unexpected victory, drunk on the local equivalent of very dark rum Sam shoves her firmly against a the outside wall of their bus, calls her a fool and informs her that if she thinks she's getting rid of Sam that easily she's got another thing coming. She kisses Iris firmly on the cheek and stumbles off to bed before she can say something far more foolish on the matter, but Iris catches her in the well of the stairs and, slightly wild about the eyes herself, kisses her properly and thoroughly. And that, appears to be that.
Sam revels in getting immersed in cultures, in trundling around planets exploring them properly rather than passing through stopping a burgeoning apocalypse on the way. She manages to involve Iris in a few of her crusades, pricking her conscience and kicking her out of her bubble of comfortable self-involvement, while Iris teaches her to see the big picture and to accept that she can't save everyone (or some days anyone at all). But somehow they both find a ridiculous joy on those terribly rare occasions when everybody lives.
They do, eventually, track down the Doctor, several times in fact. Gate crashing his life at inappropriate junctures becomes a pleasant hobby for both of them. Iris charms his new companion, a personable young man from the nineteen sixties, round her little finger and steals his cigarettes. Sam notices the way he and the Doctor fit together, like she and Iris do (like she and the Doctor almost but never quite did) and briefly wonders how she would have dealt with that if she'd stayed. Badly, she imagines, and dismisses the notion, didn't happen, doesn't matter.
Sometimes she wonders who she would have been if she hadn't crashed out in Iris's ship after the Doctor returned from his quest back on Hyspero. May be it would have been better, maybe it would have been worse. Maybe she wouldn't be any different. Whatever, it doesn't matter, she wouldn't swap tumbling round the universe in an aged London bus, mix tapes blaring, with a glamorous older woman for anything. There's a million choices they could have made, a million people they might have been, but none of that really matters. This is who they are right here and right now, and that's all that truly matters.