As far as I read, kerosene heater is over 99% burning efficiency. As far as leaving an oil film residue I haven't heard anything about that, maybe true with older heater with an improper wick. Obviously, common sense should prevail when operating carbon fuel heater regarding ventilation and leaving unattended. As far as cost, I think kerosene price is comparable to diesel @ $3-4 a gallon which last about 10-12 hr of heating. My RV is about 270 sq feet, and I believe a 10,000 BTU portable radiant kerosene heater should be appropriate (I saw one on ebay for about $110, cheap, and newer models have lot of built in safety feature) ... the Japanese have been using indoor kerosene heater safely for years and their living space is smaller or the same as my RV. The problem with propane I don't like is the refill headaches unless I get a 100-250lbs tank, refilling a kerosene heater is much simpler. One thing I heard about kerosene heater I like, is that they can put out a lot of heat and warm up pretty quickly which is a must living in Canada. Anyone actually use this type of heater in their RV?