They removed the shower walls and tub and removed the decking that was down there. They added additional legs at the back wall of the tub (closest to the kitchen) and placed a new piece of 1/2" plywood the full span of the bathtub. It is now very solid and fully supported.
Not sure why Jayco isn't stepping up to the plate on this issue. I would suggest you take it to your dealer and have them contact Jayco on your behalf.
My 2012 40' Bungalo had the same problem with the shower stall being poorly supported. My 300 lb weight strained it badly.
I took off the front inspection door and measured up some more supports made out of vertical 2x4's with 3/4" plywood pads on top of each one. It isn't pretty under there but it is now solid.
It took less time than writing an email or letter complaint and eliminated the "heartache" of waiting for approval and execution of repairs. Done and onto the next broken or poorly made thing like the low point drains.