One of the first items I acquired for my Westy was a collapsible five
gallon 'gray water' bag, complete with a hose with threaded fittings
that fit the bottom drain and the bag.  This is much like the
collapsible water carriers that are available from most camping
outlets.  It folds up very compact when not in use, easy to store.  I
believe I got it at Camping World, but I just checked their website and
can't find it.