Many folks choose to use portable electric heaters to heat their rig. This method of heating doesn't add condensation to the air and depending on the cost of propane may actually be slightly cheaper to run. Great care must be taken to not overload the wiring in your RV or the electrical system in the campground. Most parks will either put you on an electric meter or charge extra for electric heat. Use only UL approved heaters and keep combustibles away.
Fuel costs are highly variable, both because they go up (and sometimes down) and also because you may drive more or less in any given month. Fuel can cost as little as $0 per month, if you stay in one place and ride your bike around town. Or fuel costs can dominate your budget if you decide to take your RV from Florida to Alaska and back via the scenic routes through New England and Southern California — in six months!
We specifically shopped for a four-season RV when we chose our Keystone because we knew we would be spending winters in cold weather. However, some people do camp in cold weather in campers that don’t have all of these features. Here are a few cold weather camping tips I’ve seen from people who live in climates colder than ours or who maybe don’t have all of the features we have: