Still using the 1500 watt milk house heater powered by a 2800 watt Onan generator. It was in the teens last night. I did use my $19 walmart comforter last night which I rarely use, but with that I was very comfortable. A good sheet would probably be as good as those cheap comforters.
My van is insulated well except for the side doors which are right at the end of the bed. They would be almost impossible to insulate well although I have some ideas on what I'll try. Any improvement on the side doors and some improvement on the cold air that comes through from the cab and I think that little heater would do very well for me. It handles anything above 20 degrees with ease and I seldom use even a sheet while sleeping. I am typically just as comfortable (temp related) in the van as I would be at home in the recliner.
There is no doubt that the espar heaters are the best way to go and I will have one some day. The Buddy heaters are very good for heating as long as your willing to accept the risks associated with them...I am not. For now, what I'm using is working just fine.