I would Try to unscrew the V valve and the coupler that is connecting to the metal fitting. Form there you might be able to see if the metal fitting is part of the tank, or is screwed into the tank. It might be as simple as unscrewing it. Adding new gasket/ sealer and then putting everything back together. If that does not work, then the section of the belly pan in that area appears to be screwed on with flat head screws? Once that plate is pulled down you should have a good view. It looks like your holding tank is above the floor, so there must be some kind of pipe running from the tank, through the floor, and through/ out the belly pan. Oh yea clean the tank out real good before getting to work!!
Not all those who wonder are lost.