Are you sure your draining both Gray tanks?
From experience talking here, found out after several trips our RV had two grey water tanks.... One grey tank and the black water tank valves were very visible, the other grey tank valve - errr, not so much!
Look for the second grey water tank valve. Can't tell you where, don't have a TT (fiver), but the second valve on ours was hidden up on the frame, right in front of the front wheel. Seriously, you couldn't see it because it's black, and it is right on the black frame. Bet you have another one someone their hiding, which probably drains you sink grey tank.
Also, I wouldn't really leave the valve open for the gray water. It creates a direct path for the sewer gas to travel right up into your RV - really there is suppose to be a trap, but when you say you 'snaked' it out, you probably destroyed the trap - it's not a 'p-trap' like you have in the house, but probably a waterless p-trap, like shown in this video (
), that uses a soft silicone rubber one way valve to let the water out, but keep the sewer gas from coming into the RV. Those are easily torn and destroyed by a drain snake.
Good luck finding the second valve.