I never, never, ever use my potable water hose for flushing the black water tank. I have a hose for flushing and general clean up and everything else except drinking. I have run into many dump stations that use non-potable water for rinsing and washing down the area. I, for sure, don't want that in my potable water hose.
I think the check valves are installed to prevent sewer water from backing up into the hose, for those that may be dumb enough to use the same hose for potable water and flushing. I even use this nasty hose for draining gray water into a portable tank when there is no sewer connection at the campsite.
