[Latitude 28]

We have about a 12" offset to the Black Tank so its not a straight run (no pun intended) so I did investigate using a 3" pipe but did not have the room.  You're right, we love the gravity flush toilet for the reasons you described just did not like the low flow drain with the 1 1/2" pipe.  Plus, if we have a problem while on a trip it's easy to get one shipped in from Amazon for half the cost of a rebuild kit for the air toilet.