You are way off in your statement that with an almost full FW tank, it weighed 250 pounds. Water weighs just under 8.5 pounds per gallon. You had about 400 pounds of water on board, not 250. Hopefully, your other estimated weights were closer...
Don't think so though. People tend to overload commonly. I believe you were oveloaded and if that proves out, you haven't a leg to stand on poor frame construction or not.
Lippert did (a while back) release a warning about running with a full FW tank or a BW tank due to them 'falling out' of their cradles because the cradles aren't stout enough to carry them full. In as much as you didn't suffer a 'tank drop' but instead broke the frame at the front, I suspect you were grossly overloaded.