Joe, that's exactly how I winter camped when I had a Casita
. It was still worth it to have the trailer for a warm place to sleep and cook, even without full use of the plumbing.
As for four-season trailers out there, Lance and Arctic Fox come to mind as manufacturers that make them. Never priced them, but they are probably much less expensive than a new Bigfoot
. Also likely to be some good used ones out there.
Also there are a lot of creative full-timers wintering in places like the North Dakota oil patch. One usual trick is to build skirting around the trailer and put some kind of heat source underneath.