Are you working on a '73 or a '79 (your profile indicates a '73)?
Are you trying to install the new shower in the exact same space as the old one occupied, or has then entire bathroom been removed?
Inland RV seems to sell a replacement showerpan with a seat, shown below:
Inland RV Center Inc.
It says that it is a replacement for a '74-'82, but I am not sure how it is supposed to fit, as the original baths I have seen on most of the later 70's trailers have the seat squeezed into the corner of the trailer. Maybe you are supposed to cut the Inland part.