Do you have a shower valve with hose attached? If so, the hose attachment should be on the bottom, not the top. I found a similar problem with my trailer and couldn't figure out why water was dripping onto the toilet paper roll behind the shower and under the sink. I bought a new shower valve and after reading the instructions discovered that the original valve had been installed upside down. The valve is designed so that water in the hose to the shower head drains out into the valve body. With the hose coming out of the bottom, the water is able to drain out of the valve body into the shower. If installed with the hose connection on top, the water in the valve body collects and drains through the wall and into the adjoining compartment.
1985 25' Sovereign
2003 3/4 ton Suburban