So.....
Why do you think you have dual grey tanks?
How many tanks on level monitor?
How many drain valves?
I have two gray tank indicators on the level monitor which led me to the conclusion that I had two gray tanks. One indicator progressively moves to full and the other always reads empty which I thought meant that when one was full it would overflow into the other tank. I have two drain valves; one black water one gray water. I followed your lead and checked the brochure specs online and found the same info you did... So, two indicators?