Don
I know we don't stay in temps like you do but I think you will notice a difference with the window, vent and door covers. I was even thinking of cutting foam to fit between the screen door and door to cut down on the loss.
In our trailer I thought the single pane windows that were causing the draft in them but it turned out to be the metal frames. They transmit the cold right in and even using just heavy towels to cover the whole window makes a difference.
I wonder if you can get a temperature alarm so that if something fails while you sleep, you will know it before the temperatures get deadly.