This happened to me on a used unit I bought a couple years ago. After taking it back to the dealer and they, along with my wife, making multiple calls with no solution, the dealer fab'd up a brace that slides into two holders on the tongue and supports the king bed. There are also two "extra" support poles that attach to fab'd mounting plates at the forward edge of the underside of the bed, which go into the existing slots for the stock bed brace.
Essentially we have the original supports with an extra lateral support at the middle of the bed, and two more braces that support the end of the bed. It works but it is a pain to get the front bed out on setup.
I wish this problem would not have happened in the first place. This might be getting traded in...