I had my propane heater go out at -20 F and I was able to make my bigfoot livable with an electric heater. I had two big dogs so the heater had to be on the table to avoid getting knocked over, which meant the heat level only came down to the table. The camper was dry at the time though I was using the toilet. I would pour down RV antifreeze which kept it slushy but the blade for the out flow froze. I had to heat the camper up quite a bit to unfreeze that. It sounds as though they are going to use a cassette toilet so that shouldn't be a problem.
Even at the lowest temperatures I would leave a vent open as that removes the moisture. On the windows I use reflexit two layers and then plastic to hold it in place. My windows are single pane. If it gets really cold I add blankets to the windows they stay up pretty well if you anchor them on the boxes that are above the windows.
Under the bed you may want to put more insulation. The cold comes in and the mattress can get moldy on the bottom.
