COVER THOSE ROOF VENTS!!
Covering all your roof vents will make MARKED difference on getting it warm inside your MH during real cold temps.
Vents are plastic andnot sealed tight when closed. Just put your hand up to one of them. Tons of cold air comes in from them and the Heat rises and all your heat is escaping out the vents.
I have 3 roof vents on my MH. Once I covered them all the temperature in the MH changed dramatically.
AND
Close the slide at night there is NO insulation when the slide is open.
I don't ever use my furnace.
I rely on 2 ceramic heaters and can comfortably heat a 31 ft. BUT when it gets real cold you 'have' to resort to manually insulating the coach because the manufacturer didn't!~
Most of the cold is coming in from the huge expanse of the windshield and side windows. You have to do something to cover/insulate them.
Side windows in coach. I buy van windshield sun shades they fit perfectly and slip right under the day/night shades.
When you shut your coach off make sure all the ac heat vents are 'closed' before you shut the engine off.
And lastly put a towel at the base of the coach side door. Put your hand down there and you will mildly ticked off how much cold comes in from the bottom of the door.
That all said once you insulate the stuff in the coach when confronted with freezing or below freezing temps and see what difference it makes? You'll understand why your furnace runs 'non stop' because it can never keep up with all the cold coming in if you don't address the problems.