I'm not a fan of the idea because you'd be introducing black water into the grey water tank. But that said to get it to work you'd need to add a second gate valve to keep the mixed dirty water away from the cap or you'll get a nice shower when you remove the cap to reconnect the sewer hose.
There was another thread here recently about taking the sewer hose once it's full and lifting it thus forcing a wave of dirty water back into the tank in an attempt to clean out a little more. If you have 3 hands you might be able to do some fancy gate action to get the grey into the black using this method. Wear & tear on the sewer hose and increased potential for a disgusting catastrophe are the obvious deterrents.
2007 Shamrock F19 hybrid - 2010 (22 nights), 2011 (24 nights), 2012 (11 nights)
2013 Surveyor Select 235RKS - 2012 (6 nights), 2013 (32 nights), 2014 (26 nights), 2015 (37 nights), 2016 (14 nights)