When it gets cold, your first reaction may be to fire up the gas furnace. That works, but here are some other suggestions that use less propane and electricity.
Insulate Your Windows
The first recommendation I have is to keep whatever warmth you create inside your RV from escaping. Many RVs do not have dual-pane insulated windows. So you'll have to add your own insulation to keep the heat in and the cold out.
Reflectix bubble wrap insulation is the easiest and most inexpensive way to insulate your windows. You can buy a small roll of it at your local hardware store and cut it to fit your windows. Keep the custom pieces folded up and stored, because they will also help keep your RV cool during the hot months.
Use a strip a Velcro at the top of the window frame to keep the Reflectix in place, then close the blinds over the top of them. Once in place, there will be a noticeable decrease in the amount of cool air coming off your windows.
Also consider getting a custom fitted RV windshield cover for insulation and privacy. Sure, you could cut a piece of Reflectix for your windshield (which I also have for when it's really cold). A cover designed for your type and size of RV will definitely look and work better. There are many options. I personally like those that mount to the inside of the windshield. That way I don't have to go outside in the rain, dark or cold to put it on. Here's a list of windshield covers on Amazon.
No Need To Heat The Entire RV
This goes for cooling your RV as well. Only heat or cool sections of the RV that you need the most.
When using a small heater, close off other areas of the RV that don't need to be kept warm. Heating a small space will be much more efficient than trying to warm up the entire rig.
For example, try closing off your sleeping area and only heat the living area. Then do the opposite at night to keep your sleeping area warmer. At night you can also just warm the bed with an electric blanket instead of a heater.
Use a portable indoor propane Heater
There are several small propane heaters rated for indoor use and put out enough heat to keep your RV living space warm. Some need just a $2 propane canister to run, while otheres need to be connected directly to a large propane tank. No electricity is required to operate these which makes them great for boondocking.
Heaters like the Camco Olympian and the Mr Heater models have been popular for dry-camping in colder weather.
Mr Heater Buddy Portable Propane Heater - Indoor Safe
This is a great little portable heater that can run off of small propane canisters (1 = 6 hours) or be connected to a large propane tank. It's great remote camping accessory when you don't want to fire up your RV furnace.
Get a 12 Volt DC Powered Electric Blanket
A small electric blanket is probably my favorite boondocking accessory. It can be connected to a 12 volt DC power receptacle and provides enough temporary heat to warm me up. I simply put in my bed and fire it up 10 minutes before climbing under the covers. Toasty! Getting into a warm bed when the rest of the RV is cold makes all the difference.
If I get cold in the night, I hit the button to warm it up for another 45 minutes then it automatically shuts off to conserve power. The small one I have only draws about 4 amps while running. So it has very little impact on your battery.
12-Volt Polar Fleece Heated Travel Blanket
There are several sizes and designs to choose from. The one I have (pictured here) is the RoadPro. It's not full-sized and runs for 20, 30 or 45 minutes based on what I choose on the control. Trust me it's enough to keep you warm. It's powered by a 12 volt receptacle I installed next to my bed and works great.
By the way, my wife and I both have one in the RV. That way we can each manage our own heat preference and there's no fighting over the controls.
Add an Electric Heated Mattress Pad with Timer
An electric mattress pad will also keep you warm in bed like an electric blanket. Most of them require AC power to run, but here's a 12V DC powered one from Electrowarmth recommended to me by one of our members.
To run an AC powered mattress pad during the night you'd need to have an inverter running. A mattress pad with a timer and level adjustments will be your best bet. Keep it on low or set the timer when you want a quick warm up. That way you'll minimize the power draw on your battery.
Like electric blankets, mattress pads come in many sizes. You can find one on Amazon that fits the mattress in your RV.
Pull In Unnecessary RV Slides At Night
If your RV has slide-outs, consider closing the ones you don't need at night. This will reduce the amount of space you'll need to keep warm and create a tighter barrier against you and the cold.
We often close the main living room slide at night but keep the bedroom slide extended leaving us room to move around the bedroom. In the morning we open the main slide again.
Move to Warmer Climates
Finally, if cold just isn't your thing, simply do what most full-time RV owners do. Pack up and drive south until the climate suits you. But even in the Arizona desert it can get cold at night. So it's best to be prepared for anything.