losing fresh water 15 320css

Before I leave i fill the fresh water tank but by the time i get to the campsite i don't seem to have much fresh water. Its a 15 320 cs-s.  Water pump is off when traveling.  I dont see water on the ground.  In a separate issue I also notice when I'm on city water and shower it seams to fill the black water tank.  How is this possible.  We dont use the toilet.  I have to drain the black water tank after 2 showers, the gray has some water but not much as the black.  Is my fresh water getting into the black water some how? toilet bowl holds water and we only dump maybe a cup or 2 that gets in there after showering.