One of the most useful tools that you can have if you own an RV or home is a pinless moisture meter. The tool allows you to detect and track moisture in walls, under flooring, or in the OP's case, moisture behind the shower walls. It's like a wall-stud finder for water. I've never bought a boat or RV without going over everything with one of these. I also use mine around the house to check for leaks around the toilet. The meter isn't expensive, typically less than $50.
I never use the absolute numeric readout value, I just look at variations when I move the meter along a surface. The meter can also respond to other things so you have to use a bit of judgement.
The OP didn't say, but if you hear the water pump cycle every so often, even with all of the faucets off, you could have a leak in a pressurized line. That, or a bad check valve in the pump