Paul,
I am curious why you ordered a 12 speed auto for RV purposes. I figure you won't be grossing more than, say 35K tops. Could you have gotten a 10 speed auto? I think you will find that you will never need any gear lower than 4th even when starting off up a grade.
Does your new Columbia have a sleeper, or did you order it as a day cab? I think a day cab would be easier and less expensive to mate the conversion with the back of your Columbia.
onezman
http://onezman.tripod.com/whitehotrvhauler