I wouldn't consider a custom build using a manual trans. Even if you are fine driving it you're going to have problems letting others drive it. Like on a long drive with friends/family for instance. And you're going to have a hard time selling it. And, the auto shift transmissions are so nice to drive. I enjoy shifting and have driven many manual transmission cars. It sure is nice not to have to shift the rv. Now, I'm talking about the truck auto shift transmissions. Not a hydro unit like an Allison. I wouldn't consider one of those either.
'03 Freightliner FL112, 295" wheel base, with '03 United Specialties 26' living quarters, single screw, Cat C12 430 h/p 1650 torque, Eaton 10speed , 3.42 rear axle ratio