Holding tank vent exits to a fitting on the starboard side of the hull adjacent to the holding tank just below the deck. Stainless fitting should be easy to see. Mud daubers like to build nests in the vent fitting and plug it up.
DougM is right... the holding tank is in the head compartment. If you open the right side mirror/cupboard you will see the top of the holding tank and the vent tube going to the through hull fitting. By shining a strong light on the side of the tank you will see the level of fill (if there is no other gauge).
Do not leave any fluid in the tank when winterizing: these tanks split rather easily at the bottom with the expected results... been there!