Lynn, welcome to the forum!
As I was reading your description, it sounded exactly like one of the many 13' front bunk models out there with a few aftermarket modifications for off-grid use, such as a solar
set-up and a lightweight portable generator
. But when I got to the last requirement- heated and insulated holding tanks- I realized you're talking about an entirely different class of trailer.
Garden variety molded fiberglass trailers are lightly insulated on the walls and ceiling, none below (Escape offers optional spray foam underneath). Grey water tanks along with some of the plumbing hang down under the cabin. With care and a good furnace
they can be used in situations in which the temperature dips below freezing- as long as it's not too far below or for too long. But they are not built for extended winter use.
Only two molded fiberglass manufacturers make true four-season trailers: Bigfoot
. Their smallest four-season trailers are significantly larger and heavier (and way more expensive) than the smaller eggs you seemed to be describing. You can click the links to their websites to see if that's what you had in mind. EDIT- the Oliver only sleeps 3
A helpful resource for newcomers is the sticky thread Trailer Weights in the Real World
. Post #297 links to a spreadsheet of the data.
I hope that helps clarify your options. Best wishes finding something that works for you!