I went through this same struggle recently. My old shower pan/tub that was custom shaped to fit in the corner was cracked and broken, and the drain needed to be relocated for my new design.
I let the issue strand me for several months while I investigated everything. In summary, there are some RV supply places that sell shower pans in a basic square or rectangular shape, but nothing that would fit right into the space I had available. Airstream only sells one that is designed to be a side-bath set-up and goes over a wheel well. At the end of the day, I repaired the one I have using marine fiberglass on the outside of it.
If I had it to do over again, I probably would have just had one custom fabbed out of stainless. I doubt it would have cost much more than I spent in fiberglass, surface treatments, and time, time, TIME!