What about the valves that shut the water off? Have you checked the seats in them? Maybe they have been tightened too much and have a groove in them, or they might just be worn from use.
On ours there is a way to shut the head off along with the valve handles. After we use the outside shower, I let it hang for a short while to drain. As much trouble as the outside shower can be, they are nice when washing the dogs!
2009 38' Diplomat
CSM- retired, wife as co-pilot
Reka & Ali providing security (our 2 labs)