Hey,
So what you're looking for is a setup like an icehouse. An RV made for ice fishing. Many are really basic (no plumbing) but it is really common to have icehouses with full kitchens and bath/showers nowdays.
The plumbing in these has to be inside the heated/insulated portion of the house, not unprotected from the weather.
There are lots of different manufacturers if you care to research it. Here are a few - Ice Castle, Glacier, forest River Ice Cave, Yetti, Core.
For instance, in mine all the plumbing is under the built in futon loveseat thing and a false floor in the bathroom at the nose of the trailer. These are areas that are heated. In fact my furnace is under there too. (Please see the photos in my icehouse thread for pics of the layout). It's heavily insulated too. Last winter we had a week of overnight temps that were in the mid 50s below 0. My furnace kept up nicely.
~angie
Edit - to be clear, I'm not suggesting you purchase a pre built rig. I am suggesting that if you build it you take some inspiration from the design principles that the people who build icehouses operate under.
Also, in the interest of full disclosure, I have not yet used my water tanks because I have serious trust issues and I can't afford to repair the damage it would make if a pipe were to burst in the winter time. The lessening of stress more than makes up for the hassle of hauling five gallon jugs of water.
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