Overloaded or wrong truck?

Looking at it from a physics standpoint. With a lot of rear overhang and a lot of front overhang, simply driving down the highway will cause a "rocking" front to rear of that weight, centered over the rear axle. That would cause the frame to bend slightly up and down in the middle with every "rock". But even slightly over a period of years will cause metal fatigue, and eventual weakness. And enough and it will finally break.

That is not the first picture posted of trucks with large campers breaking in half like that. Would be interesting to know how many miles it had on it lugging that camper around.