I believe the story on that one was that the refer was on gas, and lit. The cooling unit popped and the ammonia gas leaked out. The ammonia gas IS flammable, and heavier than air. The gas caught and the result is in the photo. There is a chance of this happening to any older absorption refer. Maintenance really would not make a difference unless you added scraping the rust off the cooling unit and re coating it with a rust preventive as part of the plan.
The point to remember is that if you smell ammonia shut off the gas and GET OUT or the other way around, GET OUT and kill the gas at the bottles. This is the first picture I have seen of this, but I have heard of it happening in other brand RV's too. It is just a factor of the age and condition of the refer.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato