andykim

The fitting in the shower is a brass fitting, one end the spray hose goes on. The other end is 1/2" mpt

Hold the inside piece with pliers and use a Cresent wrench to remove the plastic 90 degree fitting behind the shower wall. Send a pic of the plastic fitting 

Had same problem, water leak from shower hose fitting ,close pocket door, should have access plate from wall behind sink,  water was running down inside shower wall out onto floor,  replaced moen valve with new symmons valve, they used silicon on hose fitting and shower valve,  water connections are pex with crimp rings.

Plastic elbow on shower hose connection. I replaced Plastic fitting with 1/2" brass elbow, and male pex adapter also.

No push buttons, no Aladdin system, just ball valves,  city valve is open, filled h2o tank thru exterior fill, turned on city ball valve no flow to sink or shower,  tank water system works with pump no problem to all devices 

No water thur city connection into coach, assuming check valve stuck, replace or try to repair?, how bad to replace?