Bizarre holding tank problem! It fills up on its own!

I have been trying to figure out how a fresh water leak, could possibly find its way to the gray tank, WITHOUT going down the drain. Maybe some of us responding need a clearification on what you mean by a "very short time".

Yet, within a very short time, the gray water tank is backing up into the bathtub.


We can go a month on our black water tank, which is great

A month is a VERY, VERY long time on a black tank. When you say "short time" on a gray tank.....you talking a week, a day, several hours?????????