Having a similar unit as yours, you are right. It does not take much to keep warm...at least until you drop down into the teens and/or have a strong wind that sucks the heat out of every small crack...... (I also recommend insulated curtains for the windows to add an extra barrier. It makes it seem a little like a cave in cold weather but a warmer cave than uncovered windows.
This past trip I gave up the small cube heater for a slightly larger but silent heater. I started using a small oil filled electric radiator.....it looks like s small white radiator and is electric. It heats up the oil and then radiates the heat out. It seems to have a more even heating effect and I don't have the sound of the blower all night/day. It gets very warm to touch but not so much that it bothers me or has burned me if I bump it or have to move it. It sits out in front of my kitchen area but gives me and the dogs plenty of room to go in or out the door. The dogs ignore it and since I don't spend a huge amount of time in my small kitchen area, it works for me.