A few years ago, we made some emergency heaters that I think might work for a small trailer.
We took a new one quart can, (the style that paint comes in, with the tap-on lid) and put a role of toilet paper inside. You remove the cardboard core and compress the roll, and it fits perfectly in the can. You then fill the can to the top with rubbing alcohol. The paper acts as a wick, and the alcohol burns slowly and under control. You can get about 6-8 hours from a refill, and you can blow it out and reslight it anytime.
That's the heat source, which of course, you would not want inside because of CO. But, you make a heat exchanger to be placed outside out of a larger can and coiled or snaked copper tubing inside the larger can over the flame of the smaller can. The tubing would then be routed to an inlet in the cabin wall. A small 12v fan would pull the fresh warmed air in the tube into the cabin. The warmed air intake would need to be away from the burner to draw in fresh air.
It's a simple concept that would require some experimentation, but I think it would provide enough heat to be comfortable.