+1 for the OP on calling out a real issue for boondockers.
I've thought about this issue myself, but rather than trying to retrofit an engineering control such as a manual or automatic valve, to date I've chosen to apply several administrative controls.
By that I mean, I have not yet aspired to return the first (and probably second) flush to the fresh tank. Instead, I make use of it in various ways:
- Top off the dog's water bowl - she drinks a lot of water when we are camping due to the exercise, exertion, etc.
- Set it aside in a pot to boil later for cooking (tea or cooking noodles, rice, etc.).
- Capture it and fill our fridge containers which make us cold drinking water
- Capture it in a silicone bucket for dolling back to the hot water that eventually comes out. Sometimes I crank the water heater and make REALLY hot water in it - maximum temp. Well, I can't use that straight up - it's too hot. So I used the diverted first flush water to bring down the temp of the water in the sink or bucket into a use-able range.
Just in case there's any reader scratching their head over why this would be a concern to anyone, here are my estimated numbers. There's no way that I'm going to waste somewhere between a quarter to almost a half a gallon of water during each heating event while boondocking. I only have a 26 gallon tank which I never fill with more than about 20 gallons (details omitted). It gets consumed quickly; it can't be wasted.