Chickie I purchased my Rv and the shower issue was of major weight in the decision. Maybe because I came from a toy hauler an a small cramped shower.
I would start by measuring out my existing shower paying close attention to the drain set backs from the walls. Then go see what might be available to suit your needs and will fit without causing other issues (drawers/doors opening clearance) Remember it most likely will install in pieces to enter door ways. Measure twice cut once and get the soap out. Keep the dust on ya boot