That depends on the RV's capabilities.
Mine has an "arctic kit" which has thermostatically controlled electric heaters on each tank that turn on at 44 degrees, including "elbow" heaters in the wet bay. All of the exterior access doors to the plumbing have plastic walls with about 4" access holes to get to the valves, which actually is inconvenient at times. It also has an electric fireplace that can generate a lot of heat but I've never tried it close to freezing. I also added dual pane windows to reduce condensation and noise so those should help a bit.
The Onan 5500 burns about one gallon of gas per hour under full load. The generator will only run until the fuel level gets down to 1/4 and then it shuts off. So if my 80 gallon tank is full, I could have two to maybe three days of generator. The propane furnace should help gain some time. If it works since I rarely use it except to see if it still works.
If it's really cold and windy then building a "skirt" to enclose the bottom of the RV is supposed to help a lot. You could build it out of cardboard, wood or similar in a pinch. Or even snow.
A "permanent" one can be attached to snaps for easy installation.