Your ending stipulations imply you'd rather not get involved in a permanent installation. The best heaters will be permanently installed, so they can be properly vented and not add moisture to the cabin. I have used a forced air diesel heater (e.g. Webasto or Espar) and it was great.

If you want to stay away from electric, two other choices are the Origo alcohol heaters and the catalytic propane heaters. Never used an Origo, but own a few Coleman catalytic heaters that run on 2 lb. propane cylinders. They're good to take the chill off in the morning, but I wouldn't feel safe sleeping with one on, and they do give off H2O vapor so dampness could get to be a problem if you used one constantly.