Sean, you probably have 3 filler caps on your deck
One is for taking on fuel and is in the stern area of your boat. There are two more amidships near the shrouds. Of those, one is for taking on potable water and one is for pumping out waste. There is a pumping station on the guest dock near the park in MDR. To operate the pump you remove the cap on your boat going to the waste holding tank. Insert nozzle on pump hose into the opening. Turn on the pump and hold the nozzle firmly into the opening so that the rubber gasket on the nozzle creates a seal on the opening.Boat owners are a friendly bunch for the most part and people in near by slips and around the docks often are glad to advise you.