I'm sure smarter people will be along shortly who have actually done this. As long as both gray tanks have separate pull handles and share a sewer connection, it's my understanding that you can get a vale like this:
You twist it onto the sewer fitting. When you're ready, you can leave this closed but open both gray tank handles to equalize the water between the two tanks.
I'm surprised you have such a large tank. Is your camper advertised as having 90 gallons of gray water capacity?