I researched it to death and even made test panels that I walked on for months before actually installing.
They make urethane and flexible grout that doesn't work well in a moving bus. They also make a silicone grout. I ordered some and came to the conclusion it was just straight silicone.
The quality of the tiles makes a huge difference also. High quality glazed porcelain is really hard to break but clay is very fragile.
The strongest test panel I made was 3/4 inch marine grade plywood with porcelain tile and straight 100% silicone. It allows the tiles to flex and absorb shock.that obviously won't work with clay tiles.
When I tried to pull a siliconed porcelain tile off it was absolutely stuck and would only come off in pieces with a hammer/chisel.
I've been walking and working on them for a year without issue.
Kind of a expensive way to go but I feel like it will survive.