I bought the plastic tank that mounts on the driver's side in front of
the propane apparatus guard.  It can be made to work but requires some
serious modification.  Fortunately for me, a friend from the list found
ways to do what was needed, including the following:
1) cutting a slot in the tank to accommodate the air conditioning lines
2) moving the air conditioning lines slightly
3) enlarging the hole in the top of the tank slightly.
4) designing, making and mounting the brackets needed to hold the unit
in place
Yes, graywater tanks are nice, but be prepared to handle the
installation.  Drop me a note if you want more info.
Lee
'88 Westy