Even one would likely have prevented this tragedy, though redundancy is good. Very sad.
There is another takeaway worth mentioning. Just because you are not using fuel-burning appliances inside your RV doesn’t mean you don’t need a working CO detector(s). In this case the source of the carbon monoxide was external- a generator
sitting outside the trailer. In the close quarters of a music festival, parking lot, or even some campgrounds, it needn’t even be your own.