I should have added:
Many pump-out hoses have a neoprene/rubber end on the hose, that pushes down into the deck fitting for the holding tank. The seal is made by applying downward pressure on the hose, and in theory the neoprene form fits to any opening. It's an imperfect system at best, often allowing air to enter the pump system.
A very beneficial addition we made to our pump out repertoire was a Pump-Out Adapter like this
It screws down into the threads for the deck fitting. Then the pump out hose clamps around it (after removing the universal neoprene adapter). It's gasketed and makes a completely airtight seal, and greatly improves the suction of any pump. If you don't have one yet, I strongly recommend that you get one.
Check the size of your deck fitting before ordering.