I put a tile floor in my Sister's house that had some sub floor issues that couldn't be fixed unless I ripped the entire floor out or built it up dramatically with green or rock board which would have caused a lot of other headaches (transition to wood floor etc) . I ended up using flexible grout. And while not cheap, I think it was about $50 for a ready mix quart, it's been 3 years and no cracking or popping. This stuff can tolerate 1/4" flex in the seam. But as others have said Marble seems to be a challenging choice for functionality let alone in a MH. I would probably go with a synthetic tile if not even a smaller tile on flixible backer sheets.