Casitas are well built (the components like furnace and fridge are run of the mill like in any TT, of course). The Casita doesn't have a high R value insulation-wise; the carpet on the walls helps some. The inside space is small enough to make them easy to heat and cool, though. And they are tight... not much air leaks in or out until you open a window, vent or door. Camping down to 25 degrees, perhaps 20, might work. Down to 10? I wouldn't want to, personally. You'll get a lot of condensation dripping off the windows, and the water system would need to be winterized.
The Escape, built in British Columbia by
Escape Trailer Industries, has more insulation (now standard, previously optional) and would be a little bit better, but they're still considered a 3-season TT. Bigfoot trailers from
http://bigfootrv.com/ are actual 4-season trailers and would be much better. All are molded fiberglass, quality TTs.
Mike G.
Liberty is meaningless where the right to utter one's thoughts and opinions has ceased to exist. That, of all rights, is the dread of tyrants. --Frederick Douglass
photo: Yosemite Valley view from Taft Point