The first best option would be to consult with your local trailer company or dealer to get design ideas and layout possibilities for your particular model. You also need to make sure that you don't interfere with any safety, electrical, or structural components of the trailer. You would also need to be careful about drainage, grey water, and the protection of your floor boards from water damage. We are unaware of any specific plan, as it will be dictated by the size and shape of your trailer. You may also go to your local home building and/or camping store to investigate potential shower materials (i.e. prefab stall vs. temporary curtain type).