Combining black & grey tanks

There’s nothing different about a most Motorhomes. If you only have one outlet then it’s easy. Add a valtera twist on manual valve at the place you hook up the hose too and open both the black and grey valves. The tanks will both equalize levels. Open the manual valve when you want to drain. You just no longer use the old valves. I would cut and plumb ( glue) a hard valve on if you want to do it permanently as sooner or later your going to twist that valve off by accident .. oops. I close the old valves when working on hooking up etc “just in case” to limit the amount of spill is it accidentally twists off.
I put a twist valve on when boondocking to increase the time I can go between dumping. I have 2 - 75 gallon tanks and the grey always fills faster.