Every year we live off the grid for the month of September in Colorado for archery season. Temperatures run between the 70's and 20's.
Last spring we made some improvements including bigger group 27 batteries, a catalytic heater and weather-stripping to seal the windows and doors. Our trailer (25 Safari) does have a 50 watt solar with regulator and a three stage converter charger but is otherwise stock. Even with the solar, we supplement electrical charging with a Honda generator with about two hours of daily run time.
We are back home for a break and can offer opinions on the improvements.
Weather stripping windows and doors is a cheap DIY project which offers immediate improvements. Stock jalousie windows let lots of cold air in which is mitigated with weather-stripping.
The group 27 batteries are a big improvement as well. They run the furnace fan much faster which means the furnace is not on as long in the middle of the night and the air is circulated faster.
With the old group 24 batteries, after a very cold night in the high teens, the batteries were out of juice and in the red. I would have to get up and start the generator. We have had several nights with heavy frost and the 27's still have plenty of juice.
The group 27's will fit in the stock battery box. I had to drill the rivets and take the door off to get the batteries to fit. Then I riveted the door back in place.
Last, best, (and most expensive) improvement is the Olympian catalytic heater. If it is cold and we are awake, it is on providing silent heat which does not use battery power. It is a must for hard core cold weather boondockers. I don't use it at night but others do. It is safe as long as there is enough oxygen. The manual says to crack two windows on opposite sides of the trailer.
If I wanted to spend more money, I would go with more and better solar. There is room for another panel on my roof and I would go with a state of the art regulator.
I have room for two more batteries inside the trailer. 4 group 27 agms and 100 watts of solar with an inverter to make coffee and run the hair drier would make generator use only occasionally necessary.