It will be behind the fiberglass skirting on the drivers side so you can't see it unless you bend down and look under just to the rear of the valve handles. Small knob on the end of a rod. You push on it and then turn it to loosen a stuck rotor in the macerator, it will not pump out the tanks by itself, so it the pump isn't stuck, it won't do you any good.
The emergency "drain" is really just a plug in the plumbing from the tanks and is under the skirting on the passenger side right behind the passenger door. Messy job to use.