Hello everyone,
Been a while since I visited.
I can weight in on this as well from the Rockwood pop up side.
The current High wall pop ups made by Rockwood contain a 12 gallon black tank with a conventional 4" outlet like you would see on a typical TT.
They also contain 2 - 6 gallon grey tanks. One is under the sink and one is under the shower. They are connected so they share capacity for a total of 12 gallons of grey water holding. The grey drains through a single 1 1/2 discharge located on the drivers side next to the 4" black discharge.
I hope this helps.