have you considered installing a composting toilet and eliminating the need for a black tank altogether? This is what I will do in my '74 Tradewind. Just like you I am changing the floor plan and will have a wet bath with composting toilet in the rear street side corner, and the bed in the curbside corner like the newer 23' models. This allows me to leave the grey tank where it is.
Here is my post in a recent composting toilet thread: http://www.airforums.com/forums/f446...-a-119537.html
I did a lot of research on composting toilets and it is definitively the way to go for me. There are tons of positive experiences on the web by boaters and RVers alike.