Sounds like you had a winterization problem... pull the knob fixture off the shower wall, I'm guessing the lines just screw on. That said, is this the first time you've put water to the trailer this season? Hopefully just replacing the fixture is all that is needed...
Dave
2016 F350 Lariat 4x4 FX4 SRW CC SB 6.7 Magnetic Metallic
2017 Forest River Cardinal 3850RL