I skipped the factory-installed furnace option entirely on my
Casita - Installed a
Camco 3,000 BTU catalytic heater instead.
It doesn't have a blower - So it doesn't make any noise or drain the
battery while it's running.
3,000 BTUs might not seem like much, but it runs continuously - So it puts out the same amount of heat as a 12,000 BTU furnace that runs 25% of the time.
Typically, the 1,500 BTU "low" heat setting is all I need.