As Polonious said: Know thyself. For an RV'er this means they need to know what type of equipment they have before venturing out in bad weather in the winter (i.e. Size of the battery-bank; type and amp-hour rating of their converter/charger; and the current state of charge of the battery-bank). To do otherwise may be foolhardy.
Some of these suggestions only work for motorized units.
If the grey and black water tanks are enclosed and heated then the RV is a big step towards being able to be used in harsh winter weather. If the waste tank area is not heated consider adding a thermostatically controlled heater. I use a 500 watt interior car warmer with a mechanical thermostat. There are also specialized tank heaters some of which work on both shore power and twelve volt power.
It is prudent to know how low the temperature may be. Check the weather history for a year ago for the week in the location you may be heading to. It won't be perfect but at least you may have some idea of what to expect.
The water system may be quite robust and usable so long as there is sufficient propane available to keep it thawed. I have used my RV at -37 C (-34 F). I've also boondocked for 5 days where the daily high was -24 C and in blizzard conditions so my solar panels may not have contributed much electrical charging. Do leave the cupboard doors open to allow warm are to circulate. I have another thermostatically controlled electric heater beside my water pump.
If there is access to “shore power” then adding a radiant heater (or two) may lighten the load on the propane furnace. Do keep the heaters further away from the thermostat at night. Do not add so much electric heat that the furnace doesn't run at all. That is an invitation to freeze the fresh water pipes.
If no shore power is available then a generator system may probably be needed. Most of the systems in an RV need reliable 12 volt power for their control systems—no power translates into no heat from the furnace. When running the generator, use as many electrical heaters as you can. I suggest 3 hours in the evening before bed, and 2 hours in the morning. Don't forget to have enough capacity to run the engine block heater, too.
I have a load divider which allows me to run a heater as well as the block heater at the same time. It alternates between the two. Some folks use one timer on a heater—and another on the block heater so as to not overload the electrical outlets the RV is plugged into.
Some folks have an additional outlet added through the wall of their RV. This is powered by a #12 cord plugged into a 15 or 20 amp outlet on the power pedestal. They then plug in a 1500 watt heater to help keep the RV warm. This is particularly useful to those of us who have only a 30 amp service in our RV's.
On a temporary basis, while you are awake, it is possible to use the stove top as a blue flame heater. Do not run them full blast, and never leave them unattended. DO NOT SLEEP WITH THEM ON. This may lessen the load on the batteries.
Be aware that battery capacity drops as temperatures become lower. Some enterprising folks do have heated battery compartments. These heaters operate from the generator or from shore power. Running such heaters via an inverter is a zero sum proposition at best and at worst will decrease the total run time of the system.
If the RV will be used often in harsh conditions I suggest considering a vented catalytic heater such as the Platinum Cat. ( http://www.ventedcatheater.com/
If there are not dual pane windows, or if the RV will be used in extreme cold it may be prudent to have blankets that can be placed over them at night.
I also block off the cab area of my Class C with a thick woolen blanket. This reduces the heated area and reduces propane consumption.
I have another blanket that I hang over the entry door.
Park with the nose of the RV into the wind. Wind direction may change direction overnight—but at least start out that way.
If the floors are linoleum purchase some carpet “runners” to keep your feet warmer. Don't block off the floor heating vents.
The RV stores often sell vent pillows that help to keep the heat in (and in summer time keep the heat out).
Twelve volt mattress pads and heating blankets are a lovely addition to cold weather camping and allow the furnace to be set back to a lower temperature in the evening. They usually draw about 7 amps each.
Always carry enough RV antifreeze, and the necessary tools, to rewinterize should something prevent use of the furnace. It is far better to have an ounce of prevention than a pound of cure.
Keep the fuel tank nearly full—the dash heater can be used as a temporary back up to the furnace should the propane supply fail.
I find that my dash heater works best for heating the rear of my RV if I set it on defrost while I trundle down the road.
I do not have a slide on my RV. If possible I'd recommend not putting the slide out. It will go out fine—but the next day it may be difficult to retract it.
I'm sure others will have additional suggestions for you! Have a great trip!