I am trying to clean out my Grey water tank and associated feeds on our 2004 Lunar Roadstar 720. On the bottom of the tank there is what looks like a screw-on hatch cover ( see image) which looks as though it should screw off in order to gain access to the tank. I have tried gentle hand pressure to unscrew this with no success and before I get the big hammer out was wondering if anyone else had had and dealings with this type of fitting?