This has been a “peeve” of ours, for a long time. We do a lot of winter camping (boondocking), and their poor efficiency is hard on batteries and propane. We recently did a 3 week trip, with about 3 or 4 nights in single digits. For the trip... we averaged approximately 16 pounds ( about 4 gallons) of propane daily. And....had to run the generator more than wanted (overcast days hindered the solar panels) to keep the batteries “happy”!
If someone would produce a high efficiency heater, at a reasonable price, that could be retrofitted to our unit.....I’d certainly be interested!