An RV is not easy to keep warm when it is cold out. The needs for portability conflict with what is needed to maintain and provide warmth. Insulation is usually rather thin. Supplies of propane or other energy are rather limited. There is a lot of surface area for a small interior volume.
If you are hunkered down for the winter, you can often rent a 100 gallon propane tank with a service to fill it up every month or so. You can add insulation to the windows to help reduce condensation and heat loss. You can also add skirting to take the entire bottom side of the RV out of the surface heat loss equation.
Skirting connects the side of the trailer to the ground so that the air underneath the RV is trapped. The heat that is conducted through the floor is not carried away so fast. The heat from the ground, which is often warmer that outside air, also helps reduce RV heat loss.
One favorite temporary skirting material is hay bales. A more civilized approach would be to use panels of materials from the home improvement store. 1″ styrofoam insulation is inexpensive and easy to cut to shape and should hold up for a season. Other options include OSB (Oriented strand board – Wikipedia) or even canvas or plastic sheeting. Duct tape can be rather handy with these installations but do keep in mind that a winter storm with high winds and heavy blowing snow might do significant damage to temporary skirting.
Another issue for winter living in an RV is keeping the plumbing working in subfreezing temperatures. That topic is for another day.