Norcold 12xx series fridges have had a built-in high-temp shutoff since around 2010, but it only shuts down at extreme temperatures, literally just below the level where the boiler would explode. Norcold won't say, but an independent test suggest the high-limit switch trips somewhere around an astounding 800 degrees F. (the normal operating temperature at the boiler is about 350 f.). Fridge without the built-in high limit sensor have received (via a factory recall) a free auxiliary high-limit sensor that does the same thing. The ARP aftermarket boiler manager does a much better job of managing the boiler to safe temperature range, but Norcold refuses to adopt it or even admit to its use. last I heard, they were still claiming an ARP violated their warranty and warn RV techs against installing it, or working on fridge that have it. head firmly in sand!
The high-limit sensor may prevent explosions, but it doesn't stop fires. They result from leaking hydrogen & ammonia, which can sometimes ignite if it reaches an extreme hot surface or open flame. Hydrogen, of course, is extremely flammable. Ammonia less so except at high vapor pressures. Leaks are going to happen eventually, but even moderate boiler overheating contributes to internal corrosion that leads to early failures. Norcold steadfastly ignores that except to say the RV needs to be level within their specs.
Use of helium instead of hydrogen with the ammonia helps reduce the fire risk, by helium doesn't enhance the cooling as well as hydrogen does, and too many people already complain of inadequate cooling in RV fridges. The so-called Amish cooling unit replacement for Norcold 12xx fridges has a model that uses helium instead of hydrogen. Atwood also sells a helium/ammonia RV fridge, but it never caught on.