, a wet bath is used as a method of saving space dedicated for bathroom purposes. There's no real reason you couldn't separate the two as long as the space can be reworked and you don't mind giving up some living space to do so.
You might not have to (or want to) use fiberglass surrounds - they don't really fit well in a curved trailer. If the walls and pan-to-wall joint are sealed properly, you could just Zolatone the aluminum walls like they did in the 50's trailers. Of course, the wood panel walls need to be protected somehow - we used sheets of stainless steel but fiberglass panels could be used there instead. Building the shower is actually the easy part...moving the toilet would be a little more tricky because of the tank rework that might be required. I would say if you could move the shower and leave the toilet in it's current location - it would be an easier project.
What year/model trailer are you looking at? It may help others help you with the feasibility of the project...