We've seen reports over the years of units in the exact same model-year having different plumbing. Your best bet is to have someone pour a gallon or two of water into the empty tanks, so that you can listen underneath to figure out which one is filling. Hopefully, for you, it's the gray. We can go 6 or 7 days without dumping black, but gray fills every other day, mostly because we both like our mornning shower. In order to extend gray tank life, when boondocking, we do dishes in a dishpan, then dump it into the toilet.
2014 RAM 2500 Laramie 4x4 Quad Cab w/Cummins Turbo Diesel, 2009 Rockwood 2607, and 3 'yaks!
300W of Suntech Solar, a Rogue MPT-3024 Controller, and a Xantrex PW2000 Inverter