Ed,
Travel trailers are different than S&B houses. If you consider that "modern" construction techniques in S&B houses call for a vapor barrier on the OUTSIDE of the structure (house wrap) and then when insulation is stapled between the studs, the "kraft paper lining" is considered a "vapor barrier" on the inside of the walls. But, essentially, the "vapor barrier" is on the outside of the insulated space and the "draft preventer" (non-waterproof barrier) is on the inside.
We've always considered the kraft paper liner on fiberglass insulation as the "vapor barrier" but really it's more of a "wind preventer" and "aid to support the insulation so it doesn't settle"... The "current building technology, puts the "vapor barrier" outside, in the form of a waterproof house wrap.
In Travel trailers, especially the older "rippled aluminum corrugated skin" trailers, the sheets of aluminum are slid together, forming sort of a "lap-strake" construction. Water can (and does) leak through those folded seams. So, the "vapor barrier" goes next to the aluminum skin and then, when the trailer interior is constructed, the luan panels are "vinyl coated" which also forms an "interior vapor barrier". That interior barrier is not truly "water proof" given the "careless construction and lack of precise fitting" of the wall components. I've always heard that "Molding is a cheap way to cover up poor measurements and lack of skilled workmanship"... All of that molding covers up "holes that let moisture into the walls"...
Here's one U-Tube video that details where the vapor barrier goes on an aluminum skinned travel trailer:
So, yes, there's quite a bit of difference in S&B construction as compared to RV construction. But when you get right down to the basics, houses are built with one "waterproof vapor barrier" on the outside and a secondary "water resistant vapor barrier" on the fiberglass kraft paper lining.