Having done my fare share of home remodeling, I have learned if replacing an item, it is always a requirement to have that "item" in your possession before tearing anything apart.
In my younger days and young experiences, I would measure, cut, remove, and go get the replacement or changed item. Far to many times, the item was then discontinued, or when actually in my possession, the wrong size. This led to re-work. After a few experience like that, I learned to have all the "parts" first before even starting any demolition! It's been my practice every since.
So, my advise, get the shower adaptations first before attempting the rebuild. With your experience, you'll have no problems with an RV. It's still just wood and staples or screws (they don't use nails in campers you know!)