Please excuse my lack of experience with 5ers but why would so many people fulltime in a 5er then if they are that un-economical? I did mention size difference in my post, so I am not disagreeing with you there. But, I do have two doors, my furnace has individual flexible ducts to each vent, and a slide that is half the size of my box. I am not directly comparing...I was merely explaining my experience in much colder weather while not conserving my propane usage.
Your experience is great information! I was not trying to discount it.
Personally, I have 396 sq feet, 5 slides, 14 windows, two levels and an interior ceiling height over 8 feet for most of the rig. In temps consistently between 20 and 40, even with 2 extra heaters and fireplace, we can go through a 30# tank in about 2.5 days. Luckily, we don't live where this is the case all the time, so we tolerate it. Many of our neighbors have large tanks, and I suspect it's to avoid the constant trips for refills.
Each individual has a different tolerance for cold, as well. I am OK with the coach in the mid to high 60s inside, while others may need it over 70.
I don't know if RVs are built with energy efficiency as a goal. They are cheap houses built to move. They promote that the RV's walls are "better insulated" than the competitors, or they add insulation in areas that others do not. If efficient cooling and heating were important, manufacturers would be advertising and improving more in that area, and pushing their suppliers of the heating and cooling appliances to do the same.
My goal was just to say, those with Travel Trailers seem to have more efficient heating than the large 5th Wheels, and that it's very hard to compare the two.