The Sig unit burns a lb of fuel every 7 hours on the lowest setting. That would empty a bottle in about 5 days. In really cold weather you could driain it down faster. I like the simplicity and the fact it is vented. One problem with any heater that mounts on the wall like that is finding a spot for it. I have used vented Wave cats for years, and the mounting is one reason I'd rather do something else. They all get hot! Both the Wave and the Sig are also not thermostat controlled.
The ventless blue flame heaters look attractive, but they warn against use in small confined areas. I looked all over to see if I could find out why. No documents, but I'm pretty sure I know why. Flame combustion creates CO. Most people don't know it, but catalytic heat also generates CO. The reason they can sell them as "ventless" is because they assume a certain air volume in the room and that keeps the concentration of CO below the dangerous threshold as defined by EPA. In the case of the blue flame, I'd bet you a box of donuts that the CO level in a confined area like an RV can rise above the maximum EPA spec for a space heater. The cats run below that level. Bear in mind that these levels might not make you obviously sick! I've read several technical documents on the subject including a goverment study on cat heaters done by EPA engineers. The findings of that report led me to design a ventilated box / heat exchanger for the Wave cats. I've used them in 3 different campers including a a small truck camper. They also eliminate most of the moisture.
Here's the stinker. The EPA ratings and levels for space heaters are meant to address immediate threat to life or well being. The long term effects of very low exposure to CO are not well documented. At least I couldn't find them. Knowing what I know though, I would not use any ventless heater in an RV, not even a cat.
I'm a lifelong boondocker so I've been on this trail a long time to find what you describe Doc. Everything is a tradeoff! When you talk about heat efficiency, you have to factor in all those times when you have all the windows
open because the cat (or whatever) is overheating the small space. I think a properly sized and regulated furnace
may actually be one of the best options, but that is what I'm investigating now. I even had one trailer with a custom built woodstove in it
. Like I said, I've been around this block a few times.