We keep our gray tank drain open all the time due to the fact that we use our clothes washer and dish washer a lot and along with showers, the tank will fill quickly (65 gal tank).
You are probably experiencing a bad Air Admittance Valve (AAV) located under your sink. Look for a small (two or three inches) device located on top of the drain piping. It screws off and you can find a replacement at home depot or lowes.
And yes, you are correct about leaving the black tank closed until 2/3rd or 3/4th full. If you know how long it takes to fill the gray, you might want to close its valve so that when the black tank needs draining, you can use the gray water afterwards to flush out the stinky slinky. Otherwise, the hose will still have lots of "stuff" left in it. Not good.
good luck
Ron
Ron & Sandie
2013 Tiffin Phaeton 42LH Cummins ISL 400hp
Toad: 2011 GMC Terrain SLT2
Tow Bar: Sterling AT
Toad Brakes: Unified by U.S. Gear
TPMS: Pressure Pro
Member of: GS, FMCA, Allegro
RETIRED!! How sweet it is....