As someone who has lived in our TT over a winter move insulation is key. Another thing to consider is the location of the water lines. We had the enclosed underbelly and heated tanks and let me say that insulation is the key.
While the inside was warm through use of space heaters to help reduce propane usage, we had issues trying to keep the water lines inside the TT from freezing because they ran the lines around the outside of our bunkhouse to the bathroom and then around to the kitchen. So the water lines that were behind the shower froze and stopped water from getting to the rest of the trailer. It took some ingenuity but the use of drop lights with standard bulbs (not the energy saving fluorescent ones) and the use of a heated water line to make it through sub-freezing temperatures.
There are lots of options available and you are going the right direction with your search, but remember once you decide that there are a lot of things that will need to be considered when living sub-freezing. Heating of the space is one thing, but you will need to find ways to keep fresh water and dump valves from freezing.