It helps to know what type APO you have. My Genesis has a Jabsco belt driven clutch pump, which will pump out a tank quick. It's main wear component is the nitrile impellor, which I've replaced 3 times in 3000 hours. It's a heavy duty pump, but it still needs primed to activate the flow and start the flow. It uses a prime line that is activated when the tank float turns on the pump.
Right now, mine has a small air leak at the main seal, which will reduce the flow against the vacuum. I sometimes have to go out to the van and pull the vacuum hose off for a second to get the flow started, and it will run on it's own after that. I just haven't had the down time to pull the pump apart to repair it.