If I understand what you're asking, you are trying to connect the sewer hose "barb connector" to the "Y fitting" which is also a "barb connector". You need "the other end adapter"....
You could "RUBE GOLDBERG" it and use one of these in the first photo: Or you could use one of the Thetford adapters like the second photo. They are "pricey" to be sure, and you can likely find them for less than half the asking price on Amazon at this link:
https://www.amazon.com/Thetford-4054...r+hose+adapter