I have a 2017 23RB and found the same inadequate mattress support. I added a 2x2 support wall to divide the front passthrough with the under the bed storage and built a new support to put on top of the existing flimsy mattress platform. I have acid reflux so I need to sleep at a slight incline so my added support also included an incline to place my head higher than my feet. I have a couple of pictures in my
23RB modifications album.
I now sleep better in the TT than in my bed at home.