There is a panel at the base of the shower you can remove with 4 screws and access the shower drain and perhaps the water supply lines also. And IIRC when I removed the outside shower panel it may give you access to the inside shower water supply lines. Also, under the driver side bunk is a panel facing the shower, when removed, may give you access to the water lines. Don't ask me how I know.
When you say the water is not leaking from the shower facets, but below there, could you be more specific on the where the water is becoming visible?