When it's an RV. In this case, a park model. That means the intention behind its construction is not to haul it from campground to campground from weekend to weekend, but to transport it to a site where it's parked semi-permanently, meaning it can be moved again if it strikes its owner's fancy. For comparison, here is a typical park model:
Architect Kelly Davis's creation, the ESCAPE, was designed to look like a proper cabin. All 14 x 28 feet of it.
Besides LED lighting and Energy Star appliances, the ESCAPE is heavily insulated with R28 walls, R40 floors, and R48 ceilings.