Unfortunate. I'm sorry to hear this. Sounds to me like another weak/poorly made Lippert frame.
JMO and by NO means am I defending the manufacturers, however, the basic RV suspension does little to reduce the shock transferred to the frame. MorRyde or similar center equalizers may help as well as shocks. Of course, IS would be best.
Having quality, stiff 14 ply tires doesn't help either. While they are great to help prevent tire failures they have little flex to help absorb road shock therefore transferring all the impact to the frame through the axles and springs. I really, really realized this when I got my air hitch in the bed of the truck.
I would imagine that the slide out crack resulted from the weakened frame allowing the weight of the slide-out to be shifted on its rollers resulting in a stress crack??
Sorry to hear about this and I hope you are able to find a good shop to help you with the repair since the insurance does want to help you out. The crack in the fiberglass can be repaired once the frame is repaired.
Good luck and again sorry for this misfortune.
