My trailer is fiberglass, I've camped below freezing, the furnace does a good job keeping it warm. Better yet is a small electric heater supplemented with the furnace. The smaller the fiberglass trailers are even easier to heat. As usual, condensation is the biggest hassle. Many owners stay in them for a night or 2 as they head south this time of year as will we in a couple weeks.
A guy on the owners forum mentioned just today he stayed in his last night at 5 degrees in central PA, had the furnace and a 1000w electric going with the temp set to 55. The windows iced up but that was it for issues.
