Many RVers snowbird to get away from temperature extremes, but others enjoy the cold or sometimes get caught in it. The method one uses for staying warm will depend on whether or not one has “hookups” (grid connection).
Heads up: heating an RV from batteries/solar is impractical.
The most common non-OEM propane heater for RVers is the Mr.Heater Buddy series.
Tip: you can turn the Mr. Buddy down below the LOW setting by depressing the gas knob like you are turning it off, then easing the setting back to the desired level.
Tip: you can run the Buddy on just the pilot light; this will emit a small but noticeable amount of heat (~300-500 BTU).3)
It is common to run a standard Buddy heater off the camper's large propane cylinder, perhaps teed off the stove line. Unless the line is regulated an inline "filter" (or more properly a “separator”4)) is required. This keeps oils leached out by high pressure from entering the heater and clogging the orifice.
The other major propane heater is the Camco Olympian Wave heater catalytic design, particularly the 3 (3k BTU) and 6 (6k BTU). Catalytic heaters make more efficient use of propane which should reduce costs and humidity. Catalytic heaters require extra care so their catalyzing surfaces don't get contaminated by dust or other particles. Camco says users should cover the heater when not in use, and that the catalytic pad may have to be replaced if contaminated or after a set number of years.5)
When on unmetered shore (“mains”) power electric heating is a practical solution and will conserve propane. The efficiency of electric heaters is the same (3.41 W / W or 3412BTU in a 1000W heater)6) so your choice will be made on other features.
Note: high-amp draws like electric space heaters may trip your converter's breakers. Here is a tip for finding a 110v outlet in your RV that does not go through the converter:
“With shore power connected go to your main AC breaker box and find the breaker for your inverter, it is usually 30 amps, and open it. Now check all receptacles for power. If you find one that has power it does not go through the inverter.” - garym1147)
Another note: it has been reported that some RV parks do not allow space heaters.8)
The simplest and cheapest space heaters are resistance wire heaters with a fan blowing over them. The wire is like that found in a toaster or hair dryer. The traditional design is a "milk house" utility heater that looks like it was first made in the 1940s.
Ceramic heaters encase the resistance wire in a ceramic matrix. The increased surface area means the wire can be run at a lower temperature, reducing fire hazards. It also has more even heating than the bare wire models since the ceramic adds thermal inertia. This type of heater is the most popular space heater on Amazon, with over 10,000 reviews.
These heaters are intended to generate infrared heat directly onto the target. These are highly directional and could overheat some nearby materials.
These heaters are similar to residential radiators; they provide steady and silent heat, gently heating the space through convection. These work best when the area is already warm and they can be left on for long stretches of time. Due to low temperature these may be the safest space heaters. You can also dry socks on them, which might end in tragedy with other kinds of heaters.
Applying heat directly is much more efficient than heating the surrounding space.10) Examples of direct heat include:
You can also DIY heated gear with carbon heat strips.