I just added a 24 gal gray tank under the floor, again about mid way from the axle to the rear bumper in my 70 Safari.
Same issue, where and how to drain. My main black tank is above the floor just behind the shower/toilet stall. The connector has a gray water connector but no shut-off. During the recent 4CU Vintage rally, Uwe had a connector from Valttera and I found one localy. The issue is that gravity just will not let the gray flow up and out of the tank to the main collector drain.
I am currently looking at two options, one is a stainles 12v
pump that I have mounted and hooked up to pump into the black tank (once it's empty) or just go direct with a self supported 1 1/2" drain pipe with valve for the gray tank. I'm leaning toward the self standing pipe as I tend to not make things more dependent on a pump...
My tank hangs down only about 3 inches from the bottom of the frame. I understand that Vintage TS may have a 5" hight tanks available soon. I'll keep my eye out for that...
My rear frame and new rear floor are done and new flooring is about a week away!
Let me know how you do, Stream Safe,