If it is not the by-pass vavle, check the outside shower faucet. Make sure both the hot and cold are off, otherwise the facuet acts as a mixing valve for the water system.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7