connecting grey tanks to get more capacity for the shower

Sure, we did it by making up a 3" pvc pipe with rubber elbow and 't' at opposite end with vallterra twist connectors on our 2 tank rig that had separate dumps. Simply put a valve on the end of the connector, open the dumpvalves, so that levels equalize.
On the next rig that had 2grays with one dump, we just made up a twist on valve to put on the dump, then open the 2 gray, even the black to let all the tanks equalize. Use all your waste capacity.