We have a 34' 5th wheel.......'4 season rated' with heated enclosed underbelly.
It has a 35,000 BTU furnace.
We have been in -14*F temps and stayed warm and toasty....and no frozen tanks/valves
We have stayed in freezing temps on many occasions.......never an issue.
Set the t-stat and enjoy.
I would NOT want to try cold weather camping with just an 18,000 BTU furnace unless it was in a small trailer-------22' or less.
Bigger furnace---equals more available heat-------less run time.
And if gets too hot......turn t-stat down.