This might be a bit rustic for your description but we drop in a couple times each year.
In Desert Hot Springs across the highway from PS. Hot pools with no chlorine is the best part.
I put Bills on my little truck at 100,000 and found no improvement over the OEM.
So either OEM is very good or Bilstein is weak IMO.
Next set will be KYB Monomax as I REALLY liked the KYB Gas Adjust on my old mustang.
18 btu is probably output and 30 btu is probably input or burner rating.
Get the model numbers and check the actual specs.
Either should keep you comfortable down close to zero.
I would far prefer direct heat pads on the tanks to the passive furnace heat.
As it gets that cold you will be plugged in or running a generator near 24/7.
It depends on the station. I use one card for fuel. Some pumps stop at $75 and some at $125. If I need more, I just run it again.If I am cut off that is all they get. I go down the road to swipe again.