Originally Posted by pete5016
BTW - be sure to use unsanded grout. It is more flexible.
I don't know that I would agree with that. Unsanded grout should be used for 1) grout joints smaller than 1/8" and 2) polished tile that would be scratched by sanded grout. But, unsanded grout will crack out just like sanded grout when the substrate flexes.
For a floor that is going to experience the flex associated with an RV floor, I would suggest one of the newer high performance grouts like TEC Power Grout or Laticrete Permacolor. These grouts will not scratch polished tile (since they don't contain any sand) and will clean as well as epoxy grout (though they don't have the heat resistance of epoxy). They are more expensive, but it is money well spent.