These early trailers never had separate grey water tanks. Back then it was ok to dump the gray water out on the ground. Today that is unacceptable so I ordered a grey tank that would fit where the old black water tank was.
This is a drawing of a tank I ordered from Vintage Trailer Supply with the fittings I wanted them to install.
I spaced the tank about one inch below the floor at the back of the trailer so that the fittings would line up with holes in the cross members. To hold the tank in place I mounted 2″ straps and ratchet type cinches.
I ordered a dual valve setup by Valterra. The main valve comes up on the inside at the very back and drains the grey tank and the small one drains the black tank from underneath the back bumper.
The black tank is mounted on top of the floor in the bathroom cabinet. I will install a marine toilet that macerates (grinds) and pumps into the tank.