Hello and welcome to the forums!
It is indeed the "rear hold down plate". Its purpose is to strengthen the attachment of the outer skin, U channel, floor and frame. Why it is only 36" across would have to be asked of the engineer at Airstream that designed it.
As you have seen on my thread, I used a new piece of stainless steel. Others have used thicker aluminum.
Good luck with your project. There are many great threads to read here on how to fix and prevent rear end separation.
Work is never done, so take time to play!