regardless, it IS below the Fresh Water tank fill, which is a big clue, and it IS just like many campers for the tank drain. if water is not 'coming out', then it could be plugged or the water line crimped. The water pump being on would not have anything to do with it.
Same setup as my previous rig.. 2000 Infinity.
Fresh water tank is in rear wall with gravity fill above and drain petcock below. (Rotate picture 90 degrees)
Should be a small access panel to access water pump from inside. That should also give you a view of the drain hose.
Either hose is plugged or valve broken.