I'd call
Scamp and
Casita "two-and-a-half" season trailers. They're not really intended for winter use in cold climates. They have minimal insulation, exposed plumbing, and single pane
windows. That said, if all you want is a warm, dry place to sleep, and given a good supply of
propane, they'll do. Fresh water tanks and black tanks are inside the cabin, but grey water tanks hang below the floor.
Escape is more of a three season trailer, especially if you order the optional underbelly insulation and double pane
windows. Not sure about tank location, but I'm guessing at least some of the plumbing is outside the cabin.
All the above trailers would be used dry in the kinds of temperatures you describe.
For a true four season trailer- more insulation, double pane
windows, fully enclosed holding tanks and ducted heat- look to
Bigfoot 2500 series trailers. They're very well made, but heavier and more expensive, as you would expect. The smallest is 17.5'. You might actually be able to get a hot shower in winter. Of course, you have to winterize it between trips.
Here's a nice one that may still be on the market:
2008 Bigfoot 25B175GX Xtreme | Southern, IL | Fiberglass RV's For Sale