I know this isn't really what you are asking/looking for, but when I am in
an area where grey water disposal is an issue, I carry one of those large
capacity oil drainage tanks available at your FLAPS and lay it on the ground
underneath the sink drain.  When it's time to dispose of it, I put the lid
back on and carry it to an appropriate spot to drain it.  It's not the best
solution, but it works okay for me for now until something better comes
along.  Last weekend I saw a popup camper using one of those portable water
tanks on wheels for the same purpose.  It had more capacity than my setup
and seemed to work out well for that person.

Steve

89 Westy