Looked at tile today. The 12 x 12" onyx is so marbled I doubt it would do well in an RV. But they make a really beautiful 2" x 4" onyx mosaic. Laid with a flexible, urethane grout, I think it would be safe. Here's a couple of photos:
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This is 3" x 6":
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White Onyx:
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Red Onyx:
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I asked the tile guy about a lightweight backer board. He showed me about 3 different kinds, each getting lighter. Then he mentioned this:
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...which weighs about nothing. It's a product by Schluter called Ditra. They call it an uncoupling membrane, and it's pretty interesting. Their video
https://www.youtube.com/watch?v=f6FyWs2WZ1khere says that traditional tile floors were laid on an underlayment that rode on a layer of sand:
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...which allowed it to float and not crack under stress. Here's their diagram of their underlayment doing the same thing:
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And here's the installation:
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Maybe this is old news to you, but it's pretty cool to me. It ought to save me a few pounds and protect my tile against stresses on the road.