IDK? My 2000 3500 van with a 350 gets 12mpg on a GOOD day. Same as when it was new. I don't know what you could do to get much more than that. If you don't drive it a LOT, I don't think you will ever get back the money you'll put into it. But I'm not the best one here to say either.
Young enough to learn ... too old to remember.