If nothing has been altered on you boat from the factory, the holding tank vent is a hose that comes out of the tank (the bilge floor) just forward of the two PVC pipes that are the inlet and discharge pipes. The vent hose runs up through the hanging wet locker just forward of the chart table on the Stbd side. It then runs up under the hull to deck bulwark aft to the transom. On the stbd side of the transom there are two vent fittings just below the teak capping. One of those is for the holding tank and the other is for the stbd water tank. If you fill your water tank, you should hear and/or feel air being forced out of one of those vents. The other is the holding tank. When you pump into the holding tank you should get air coming out of the vent. When you are having the tank pumped out, you should be getting air being sucked into the vent fitting. If you are having trouble pumping the holding tank out, the most likely culprit is the vent.
FWIW, mine is plumbed slightly different. Waste vent runs to the aft starboard side as described, but then crosses to the port side at the transom.. The Port water vent crosses to the starboard side. So my 2 starboard vents are both the water tanks, and my 2 port vents are fuel and waste.
An issue might be that the tank is very low, and the pump out at the marina doesn't have enough lift. I struggled with that, having to fill a bucket with water, and pump that to get the pump to prime, and still struggled to get pumping started. It seemed like the vent was clogged, but I used a pressure water pump and pumped water into the holding tank though the vent, so I know it is now clear.