You don't mention if the leak occurs when you NOT on city water, but using you fresh water pump & tank---does it? i would look for pipes not tightened, etc. Some rv's have a round access panel you can remove to get to the shower valve fittings. if yours does--check there, perhaps you will have to remove the shower water control.
AND make sure you are using a water pressure regulator
RV's water pressure are usually not meant to be above 55psi---doing so may damage the water pump check valve and cause leaks, It's not unheard of when connecting to hi psi "city" water---the pex barbed plastic fittings have popped out of the pipe!