My friend was in the market for a small, lightweight and inexpensive trailer. A lot of the models didn't include shower surrounds, just like the OP noted. I know the wall board is designed to handle some water... but the problem that I have is that those wall boards are not watertight at the corners where they meet or at the bottom where they transition to the floor pan. At some point, water is going to get somewhere that it shouldn't and eventually cause issues.
Its a cost saving feature, potentially a weight saving feature... but I encouraged my friend to save costs and/or weight elsewhere. He bought a camper with a full plastic wall surround.