I prefer a true manual, personally. Unless I end up short a leg, I would never have a full auto, even though there are convenience arguments in favor, especially the co-driver issue. I am simply not inclined to take the risk of not having a clutch, plus Fastlap's report on his experience with the World tranny is not confidence inspiring. One cannot make an appointment for an emergency, and a 15 second delay may be life threatening in some circumstances; 15 minutes is not acceptable.
My truck conversion will replace a house, for a lot of reasons, although it might spend a bunch of time parked on rural property. I get the tax deduction; I can get a wild hair and take a long weekend in Glacier or someplace like that; having seen the social aftermath of Katrina and knowing that mother nature can do lots of things having bad effects, I want an escape pod. (I do legal advising for law enforcement and public health here, in addition to other duties, on both the civil and criminal sides. Anyone who thinks that the social net will ever work better than the C-F in New Orleans is delusional.) I have big (protective) dogs now, and will have them as long as I am physically capable of doing so. They are too big to fly as other than freight, and the airlines and DHS/FAA tend to disapprove of my normal load of safety equipment. :-0
There are two choices for me, which will interact with other aspects of the specing to be determinative as to make of tractor, etc. If I ended up going with a manual, it would be a 13 speed Mack, with the extra low first and reverse for working in tight places without slipping the clutch. I have previously described my analysis of why a 13 or 18 is the winner for cross country, especially the ability to split the top gear in response to states which restrict MHs to much lower speeds. Obviously, that means a Mack tractor, with one of their high torque rise on-off road powerplants. Realistically, that would be enough torque for anything likely.
Otherwise, it would be the auto-shift Fuller 18. I think vapoppa is on the right track with that. In that case, it's a Western Star tractor, with the C15 Cat. I recently learned that the brake saver, which I would insist on, cannot be had on a Multi-torque motor, so I would probably go with the 475/1850 version. (Better to have more power and capacity and not need it than to have to mess with it later.) The Fuller service network is pretty broad, and as they move more of these auto-shift and full auto trannys into OTR trucks, service will get easier and less necessary.
I am not worried too much about re-sale. The auto-shift is not hard to drive as long as you can press the clutch a couple of time a day. In addition, the truck and conversion will be speced in a manner consistent with long term living and travel, being an escape pod, etc. It is not a goal to sell it. I will have generator capacity, solar panels, LOTS of marine storage batteries, NO PROPANE, household (not RV) appliances, lots of water capacity, etc.