What i do is put the heater on high for a few minutes to get it red, then scale back until i find the min necessary to keep it from sputtering or going all blue. When you get down in the teens though, just gonna have to resign to running it between the high and low. Its very cold at the floor level so it needs more juice to stay hot.
I had a rusty hole filled van too, and the thought the same thing as you. I could actually feel a 'breeze' down there most times, but my digital co detector told me it wasn't enough. You need to crack each window an inch or one window 2 inches if you dont have a roof vent (with a roof vent in the back near the heater, i think you can squeeze by with one inch). Get the little rain guard things over the windows if you can, they are awesome. My current van doesnt have a roof vent in yet, so i use a 2 inch gap on the passenger side, and will occasionally crack a rear window to regulate temps.
Here is me testing CO in that van with the roof vent and rusty holes
Oh yeah, on my buddy heater there is this little metal pin thingy, i dont know the word for it. Usually youll notice it glowing like a hot coal in the pilot light when you need to do this. Clean that off with a q tip when its cooled off, black crud will come off. This seems to help a little as well.
If you use a 20lb tank with the hose and filter, sometimes the filter will freeze up on me (it collects water/oils/etc that would go into heater) and it makes it go blue/sputter or gutter out sometimes, as i think the propane is slowly being chocked off. If its real bad the heater wont stay lit more than a fee minutes, i switch to my emergency 1lb bottles or swap a warm filter in.
