swapped out queen matress for king

[thor]

I bought a 2014 silverback 29RE late last fall. its brand new and we are greatly looking forward to trying it out once the snow is gone. Right now doing lots of modifications one of which was making a deal with the salesman who allowed us to swap out the queen matress for the king. the king is 12" wider and fits fine as its a factory option anyways. The particle board that it sits on however now needs to be extended to support the matress properly. I dont know however if the width of the box it sits on is wider on the king models and im worried the board may just sag if i extend it 6" more on each side. If anyone with a newer silverback that has the king could measure the width of the bed box and let me know id greatly appeciate the info. Thx all.