That last illustration David attached shows how a flexing of the cradle (the arms springing up and down) shows up on the rear cap as stress fractures. The attachement to the upper structure of the coach by the arms contributes some strength to the cantilevered arms, but you should note that the arms are substantially inboard of the outer vertical members, so as the engine cradle arms are flexing up and down, there will also be some flexing up and down in the horizontal members which extend outward from the cradle to the rear vertical members. But the bottom line is that adding a ton of weight on the hitch exerts a lot of force on the entire rear structure of the engine cradle arms and upper structure of the coach. I strongly suspect a 40 foot coach can tolerate a whole lot more tongue weight than a 45 foot coach.