Well besides the fact that you by now know, that you should leave 'both' tank valves 'closed' when parked whether it be a month or just a day.
Because you need water to move sewage and water to move food stuffs, grease etc from the gray and blacks OUT of your tanks. Otherwise it just builds up on the bottom of your tanks. Not to mention that you are getting all and everything that is in that sewer system sewer gas and flies right back into your tanks. And my guess its fly and/or mosquito larvae too.
I'd clean both tanks out with some bleach. Fill both tanks up with water to just about fill and pour bleach down the toilet and the sink. Let it set for awhile and dump both tanks.
When I clean out my black tank I just run the bathroom sink (since it dumps to the black tank) untill black tank is full with just water and then run the kitchen sink to fill gray.