If you use a polymer modified mortar and grout (maybe toss in some latex additive) your tile or grout won't "crumble the first time you moved". That is a perception that is unwarranted. As long as the substrate is solid, conventional tile installation materials work well.
FWIW, I have a tile entry area and a tiled surround for my space heater. The installation is almost five years old and is just like the day I finished it. This is maybe the fourth or fifth tile installation in an RV that I've done and maybe # 2000, overall.
Nice work from the OP, it looks night and day better than what he had. If you were closer, I'd set up the tile saw for that cut around the electrical box. You can also cut it with a diamond wheel on a small grinder. Tape the back and front of the tile before cutting and grab it firmly to avoid breaking (reduce vibration). There is nothing written in stone (pun intended) to say that you can't cut a grout space where the tile broke if the cut is difficult to make.
