As an older couple, my wife and I have been camping and pulling trailers nearly 50 years now.
In thinking back over the years, I believe the number one failure we have seen on trailers/fifth wheels are tires.
Next it seems broken springs rank number two, along with wheel bearing failures, when it comes to towing travel trailers/fifth wheels.
Perhaps others can mention other failures that we can use as preventive measures; not only for our family's safety, but for others traveling as well. We can all benefit from safety knowledge.