I installed two 12 volt water valves in my trailer, one at my kitchen sink hot water line and one the bathroom sink hot water line and plumbed them both back the fresh water tank fill pex pipe. To operate the valves I installed a 12 volt push and hold to operate switch. The install took about 4 hours and a six pack and was not hard at all.
Now at the kitchen sink I push the switch for 11 seconds for hot water and the bathroom sink takes 7 seconds. It doesn't sound like much, but at the days end I probably saved about 2 minutes of water going to waste, which when you are boondocking is a lot.
Would I do it again---In a heartbeat.
The information is out there, all you have to do is let it in.