We had a Thor Wanderer and the shower pan cracked beyond repair. I contacted the mfr of the pan (I think Thor may have told me who made them) and literally purchased the last one on their shelf. It was no big deal to replace. Mine had screws around the perimeter, then you had to seal everything with caulking. In theory you could use any brand but there are no size/shape standards and they vary a lot from brand to brand. Repair would be my first approach, followed by getting an exact replacement. Beyond that you might need to get a whole shower kit.
2003 F250 7.3L Powerstroke Crewcab longbed