I have a 2010 as well. My grey is the left handle. My normal procedure is to empty the black tank and then empty the grey. Quite often, the black water backs up the sewer so, when I release the grey, the grey water fills the sewer hose and drains slowly. Is your hose full of water or is the grey tank trickling out? If its just a trickle, you may have an obstruction or faulty valve. A wire might help you probe the valve opening and, if that is fully open, you can use the wire to push deeper to the tank orifice. Just be thankful you dont have to do that on the black water tank.