It's very difficult to give you a good answer, your profile is blank and you don't have a signature that tells us what type of RV you have. I can tell you what works for me though. I have a mid-sized Class C.
First off I travel with the rearmost roof vent open, between an inch and wide open and the heat set to dash and pointed back. This points the hot air towards the vent which is vacuuming it further back. I can keep the entire coach pretty warm by just this method. That means the whole coach is much warmer when I stop for the night.
By the time I stop the coach batteries are fully charged and the interior is warm. I set the thermostat to about 60 deg. and get under warm blankets. The furnace doesn't run much overnight.
Early the next morning I'm up, the engine is running, making heat and recharging the coach batteries.
I've overnighted in some pretty cold temperatures, not a single problem yet.
BTW, your furnace runs on 12 volts, so as long as you have decent batteries and limit run-time you should be fine.
