The interference layer product will work but I'd save myself the expense and trouble of using it. About the only way I'd use it is if I was forming a shower pan and could use it as the protective layer for that. A formed pan in an RV is almost prohibitive.
FWIW, I've installed tile in a few RV's. My current rig has a tile entry and the area around the heater is tiled below and around the heater. None of it ever failed. I've installed it directly on wood subfloor and over cement board. The trick to getting a good bond is to use good thinset. In a shower area, I would use a good quality modified mortar (thinset) and call it a day. Modify your grout with plenty of latex and all will be good.