These folks have a huge selection of tub and shower bases and surrounds designed expecially for RV's, made by the companies that supply the RV manufacturers, in the standard sizes RV's use.
http://www.rvpartscountry.com/RvShowersSurroundsI do not have any experience with them but found them when I was looking for showers to replace a tub in a travel trailer I was considering.
Charles
'03 Ram 2500 CTD, 5.9HO six speed, PacBrake Exh Brake, std cab, long bed, Leer top and 2008 Bigfoot 25B21RB.. previously (both gone) 2008 Thor/Dutchman Freedom Spirit 180 & 2007 Winnebago View 23H Motorhome.