So the question is-- Where are you putting the gray tank?
I slung mine under the floor, between the cross-members. The shower and rear bath vanity drains tee together under the floor, run through the openings in the frame cross-members, and dump into the top of the gray tank. Essentially, the drains run at the same level as before, but rather than running on the previous open path which was acceptable in 1963
, now it is collected in the gray tank.
Does that help at all? If not, ask more questions!