http://www.trailerlife.com/rv-gear/rvs/60s-flashback-riverside-rv-white-water-retro-150/something like the above ? It shows a 5 gal black tank which is a good start, the gray water would use, as you said, a portable tank which is ok but you need to realize its limits. Go small enough for yourself to handle the weight. When i did it, i preferred a smaller tank that i dumped more often. You could buy a 7.5 gal water jug from walmart and use it for gray water. It would be easy to handle and inexpensive. $15 opposed to $100 or more. Just mark the jug so you or anyone else doesn't mistake it for real water . :E