Well that looks like a familiar problem. Had the same issue on my 2008 international 25’ with rear corner bath....
I outsourced the repair to timeless travel trailers here in Denver. It was extensive labor - paid $6k for it to be fixed right.... there is a high risk of the shower cracking / breaking on removal. Fortunately timeless got mine in/out without damage. New shower is $2k from airstream.
Everything in the rear 1/3 of the trailer needs to come out to lay new sub-flooring. Everything.
See post 5 here: