My parents used to have a 5 gallon collapible gray water tank
for their moterhome about 20 years ago.  It was blue and looked
much like the ones pictured in Camping World, except that the
plastic was thin and flexible.  It was called a "Covered Bucket",
but I don't remember the manufacturer.  Next time I see them,
I'll see if they've still got it, and if it has a manufacturer labeled
on it.