I had the same idea as you did Lynnmor, but what I did was use a 120 volt operated water valve and a push to operate switch by the sink. It works great and doesn't waste so much water when out boondocking. I have to hold the switch 7 seconds when in the bath area and 15 seconds in the kitchen.
Can you list the make and model of the valve you used. And if possible a quick drawing of your plumbing?
This is something I want toincorperate into my trailer before the season. I boondock a lot at race events and want to conserve H2O as much as possible.
Thanks