Had the same issue with our Flagstaff HW Popup. Also totally confused as to why they didn't put the same type of connector on the grey as the black tank. I wanted to find an adapter to connect to the grey outlet to make it the same standard type connector as on the black tank, so I could flush the hose after dumping the black water with the grey water. After fruitlessly searching I ended up making my own.
I went to Lowe's and found a PVC male threaded connector that was the right size to fit the grey outlet on the camper. I also found a rubber clamp-on type coupler to connect the PVC piece to a connector sort of like this one that I found at my local RV dealer parts store.
Valterra Sewer Fittings - Product - Camping World
That way I just screwed on my adapter that I made to the existing fitting from the camper and I was able to use the same dump hose for both tanks.
I wish I had a picture of it for you - heck if I still had it I'd just send it to you, but I left it in the camper when we traded it in.
2012 Flagstaff V-lite 30 WTBS
2011 Ford Expedition
4 kids and a hubby & never enough time to camp!
2011: 17 nights 2012: 26 nights 2013: 46 2014: 20 2015: 16