J have a 2011 3185 and the same thing happened to mine closer to the bottom of the stairs but in the same general area. Take it to your dealer and they will go in from underneath and find that there is a trough from the sink to the gray water tank that is not supported. They had to repair mine and installed a 3/4" piece of plywood and glued and screwed it from underneath without cutting out the flooring. That was 2 yrs ago and it still is good. The trough was for the sink drain. I think they had to drop the grey water tank a bit because it was installed too high.
Rick