We've done this type of conversion in the past. See:
http://photos.transportdesigns.com/v...arter/Kentucky . It was a 53ft double drop deck Kentucky semi trailer. We stripped the interior wall liner and converted the front 12ft "attic" plus an additional 4ft of the cargo deck by extending the attic deck over the main deck. Unfortunately, I have no photos of the rear area that had (2) elevators for storing the customer's pulling tractors. You're going to have to register it as a semi-trailer however. Some states (Montana likely) may let you get away with calling it a travel trailer, but I wouldn't want to be on the same highway with someone who doesn't know what they are doing with an air brake 65,000lb GVWR semi trailer.
We use the commercial plumbing and appliances that geofkaye mentions, although I've not found them to be "cheaper". Kohler and Moen Fixtures, Kitchenaid, Fisher-Paykel and Sub-zero are much more expensive than "RV Quality" products.
Dometic Suburban and others have decent warranties, but if you're doing it yourself, you have to find a source for them. The "source" may mark them up enough that Lowes or Home Depot is going to be cheaper depending on the appliance or fixture, but generally you're not going to find a 120VAC/LP refrigerator at Lowes.
I've read in many trade magazines how down Elkhart is, but we've been blessed in serving a nitch market. Super high end, super custom and staying fairly busy year round. I wouldn't want to be a "production facility" these days with the economy in a crash-and-burn mode. It'll all come back though, with fewer survivors. I'm an optimist - always have been. The companies that can hang in there for as long as it takes will have a few good years without as much competition until the startups start up again and flood the market with price conscience products.
God Bless.