Very much “the smallest room”, we weren't sure how small we could get away with, but our spy mission to the Caravan & Motorhome show (we sat in a LOT of bathrooms) gave us the confidence that a pretty small space was fine for a porta-potti and emergency shower.
Bracket to stop the porta-potti sliding around, actually re-uses a hole in the floor that was holding the LPG tank down:
And voila, bathroom complete for now - as we were a bit behind by this point we didn't work out a shower, but we had the loo which is more important in an emergency! Shower curtain was cheap, mounted on a length of net curtain wire. Ceiling bracket picks up on existing holes in the ceiling. B&Q value loo roll holder. Wash your hands in the kitchen sink - why carry two sinks and plumb all the extra stuff in?