There are quite a few class C's that have enclosed plumbing, including the tanks, without having a continuously enclosed underbelly. If the goal is for it to be usable in cold weather, this is quite sufficient if done properly. It's pretty hard to enclose the underside of a truck chassis and still allow for the drive shaft, axle, exhaust, etc. to go where they need to go (at least without making the unit a lot taller than it otherwise would be). A trailer has a lot fewer odds and ends to worry about, so enclosing the whole underbelly is a good bit easier there.
Phoenix Cruiser has nearly everything you want except for the cabover bunk and are quiet well built units.
