If your grey tanks empty into the same outlet, you can get one of these
. It fastens on to the outlet; make sure it's valve is closed. Open both grey tank valves and their levels will equalize, assuming that the grey tanks are installed in the trailer at the same level. When you dump, you fasten your dump hose onto the twist on valve and dump as usual. I usually remove and rinse the twist on valve after dumping until I need it again. Note: I would think that including the black tank into the equalizing procedure would not be a good idea.
Also, I've read of folks who use a small pump to pump grey water between tanks.
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
2011 Jayco 28.5RLS; 2008 Chevy Duramax; Pullrite Superglide Hitch