I had this same identical problem- I fixed it myself by adding 30% more screws & putting slightly larger screws back in. I used stainless steel screws to get the higher tensile strength=greater strength. I've never broke another one since making this change.
Newmar tells me this happens when the slide motors are slightly out of sync. They recommend re-syncing the slide motor controllers. Not difficult but hard to get access to them - up inside frame rails -for an 'old man' such as I!