If you look at my floor on Viewrvs.com (Fred's) 1994 360 Classic pusher. I did all of the tile.
Marble floors, glass tile on the walls, bath and kitchen. There is only one way to do this. 3M RTV (expensive). This RTV is made for boats, waterproff and is the only thing that will work. Second, use only flexible sanded grout. I have had this in the Coach for a long time over any type of conditions, no problem.
Remember one thing, the classic has a Spartan frame that is very substanial, does not flex.