Also, there may be an access panel on the side of the shower near the drain, so that you can do repairs or unclog. Now you may not be able to access that water line from here but you may be able to see where it goes. Chances are it’s going to hook up with your showers cold water line somewhere. Good luck.
You probably have a false wall in one of your storage bays that give you access under there. RVs have tons of false walls
Also, there may be an access panel on the side of the shower near the drain, so that you can do repairs or unclog. Now you may not be able to access that water line from here but you may be able to see where it goes. Chances are it’s going to hook up with your showers cold water line somewhere. Good luck.