Since the plumbing standard for gray waste lines is only 1.5", I see no problem. However, it seems odd they necked it down to a smaller size for that short distance, when they know a 3" outlet is required for a standard sewer hose connection. Of course, it is equally odd that they put a 3" outlet in the gray tank. At a guess, I suggest that they are using a tank that is also used for black waste in some other model and they decided to build all the tanks with the same size drain port and adapt to whatever model they get installed in. That's the sort of tradeoff that production engineering makes to reduce the number of parts they have to deal with.
Former owner of 2004 American Tradition
Home is in the Ocala Nat'l Forest near Ocala, FL