Man, I have seen so many bathroom builds with minimal to zero water proofing of the substrate it drives me mad. A vinyl liner at the bottom at least would be a good start. There are such easy options now to use that I can't believe people don't use them. Redgard or Aqualock just paints on and cause a non permiable membrane. Heck, it even changes color to let you know it's dry.
It doesn't matter what you use for the grout, including the flexible grout, with all of the movement of a bus, your going to have water leakage at some point. You may not see it, but it will happen and it will be nasty and moldy and gross.
This happens in normal houses that don't bend and flex, now compare this to a moving vehicle. I'm not at that point on my build, but I will be using Redgard when I am (because I'm familiar with that product). Anyways, just my 2 cents.
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