The words you quote there look very familiar- as does the link above to a valve that simply screws on to the hookup- no modifications required.
In one of our early camping trips we had Water + Electric, but no sewer nor a tote. The gray tank filled up fairly quickly (and that's not unusual) while the black tank was much more empty. This crimped our style slightly on our last day.
So, I bought the valve at the link above- so that if we ever wound up with a full gray tank, we could use it to move some gray water to the black tank.
So far, the valve has worked perfectly by somehow keeping us from being stuck with a full gray tank and no sewer...LOL. Someday, I may actually use it as intended. :R :B
...and warning, these threads often have a hot debate break out between folks who are strongly outspoken and poor listeners about their phobia that the black water might touch their gray tank, and are unwilling to acknowledge that, soap + body oils + dirt + food particles + naturally occurring microbes makes for very nasty, germ-laden brew of stuff all by itself...