We dry camp, 3 up, for a week or more. Here's what we've done.
Get an add-on knife valve and install it in the dump line. That way, you can close the added dump line valve, open the gray, and then open the black for a few seconds. The gray will equalize with the black (shut the black first, before it quits running) and give you about another 20 gallons of real world capacity.
I'd recommend making sure the dump hoses are hooked up before you pull the add-on, though - or you'll have a mess, even with both the black and gray closed.