IMHO, the fresh water flll (and vent fitting which I see no one has mentoned) is located in a (better) protected place to lessen chance of contamination. It is a potable water supply by design. In my model it is located on the bridge deck off to the starboard side.
Can you imagine if something else was entered into tank that has a fill port looking nearly identical as the head waste tank pumpout, gas fill, and 2 stroke oil tank fill ?
Use a dock hose with the regular pistal grip or twist nozzle right into tank opening, none of the fire house type nozzles. Use a large funnel when adding antifreeze or treatments. Check O ring for damage and replace every 3 yrs or so to maintain a positive seal.