Porthole, you are making a serious error by connecting the same water in let to the Black tank flush and the city water inlet. Excellent chance for cross contamination. It may look nifty but bacteria don't care.
Not worried and not going to happen as long as I own the trailer.
The picture is an old picture, and there have been a couple of changes. But even before the change, never worried.
There is a bronze one way check valve for the waste tank behind the panel (and not a cheap RV version)
There is a bronze check valve for the fresh water into the coach behind the panel.
In the picture, the mode to flush was open the black handled valve to pressurize the pipe then the yellow flush valve. There was always positive pressure in front of the valve.
When filling the fresh water tank, that line was always flushed first, just to get rid of any stale water in the line.
My fresh water tanks do not fill through the fill pipe, they are filled through the two tank drains. So when filling those tanks, and flushing the line first, the lines are clean before the water gets into the tank.
I also sanitize my water system twice a season at a minimum, which is probably twice as much as most campers do, since many never drink the tank water.
We use our tank water all the time.
The change is similar, but the fresh water tank fill was moved from where you see it here to after the filters on the inside of the basement.
The only thing I would do different if I was to start over, would be to include an additional check valve before the 1/4 black handle valve.
That will be on the next RV
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And of all the concerns I have about bacteria from waste, I would say my biggest concern is pulling into a site after someone just left.
We have no idea how "sanitary" some people are or are not.
And I always bank on - "the are not".
just backed in to our spot. 6 days of this view. 
