Merging the galley and grey tanks together

[JFM-jr]

I am currently looking at a 5th wheel that has seperate galley and grey water tanks (50 Gallons each). The issue I have is I bring my teenage daughter and she usually has at least 2 friends with her. Dry camping on the beach turns into a grey water being dumped every day and refilling the fresh water as my current Travel Trailer has a 30 gallon Grey and 40 fresh. This 5th wheel would fill the 50 gallon grey long before the galley. This 5th wheel has a single dump port coming off the bottom and i'm wondering if the following would allow me to fill both the galley and grey together. I would install one of these on the outside where the sewer dump is
and keep it closed then open both the galley and grey valves on the camper allowing the galley to be backfilled into when the grey fills. When i'm ready to leave I would close both grey and galley open the outside valve then dump the black,grey/galley. Any thoughts?? Just wanna be sure i'm not overlooking anything....

[Sportracker]

Couldn't see the pic on my phone, but what you described is what I did with my Sunseeker ; put a blade valve as the last item on the sewer plumbing. Got it at wally world. I open both the shower and the sink valves and let the grey tanks mix. Seems to work great. Also gives a little extra security when removing the cap to dump knowing I have an extra bit of security holding back any water seeping around the other valves

[BiigDaddy]

I did this. I had a gray for my outside kitchen and one for my indoor plumbing. I cut both gray lines and merged them with a Y-connector. I no longer have two dump valves and my gray capacity doubled. A must do mod!