Most RVs are equipped with insulation that has a rating of R-7. This rating offers the lowest amount of insulation and is the minimum that RVs are required to be equipped with. Not only does R-7 insulation let heat escape, but you'll spend loads of money on propane trying to make up for this heat loss. The higher the R-value of the insulation, the warmer your RV will stay on the inside. It also helps hold cold air from your A/C unit in during the summer months. When shopping for an RV and comparing R-levels of insulation, consider where you're going to be doing your winter camping. If you're heading down south where you won't be trying to stay warm in freezing temps, you may not need insulation with a high R-value. However if you're doing true winter camping, say here in Michigan, you'll want a much higher R-value of insulation to help combat those cold, windy days and nights. On some high-end luxury units, such as this Entegra Coach Aspire 44B, you'll find insulation that is as thick as R-33!
If you're planning to use your RV in the winter, make sure it has a good furnace. You'll want one that has at least 30K BTUs, like this Forester LE2251S Class C motorhome. If you've found an RV that you like but it only has an A/C unit with a heat pump, it'll fall very short in keeping you warm in below-freezing temperatures. It'll work fine on chilly fall nights, but not when the snow if flying and the wind is whipping! Just like with the R-values of insulation, look for higher BTUs on the furnaces for better heating capabilities. Also, ducted furnaces heat more evenly than just a furnace unit.
Heated and Enclosed Underbelly & Valves
You will see that almost all RVs come equipped with an enclosed underbelly. This serves to protect the tanks and lines from getting damaged as you head down the road and maneuver in and out of campgrounds. If it's not heated, however, you're going to end up with frozen tanks, lines, and valves. A heated underbelly usually has a duct that comes from the furnace to fill the enclosed underbelly with warm air to keep tanks, lines, and valves from freezing up. Another cold-weather feature to look for is tanks with heating pads. like on this FR3 32DS Class A motorhome. These apply heat directly onto the tanks to further help keep them from freezing.
Glass isn't exactly a great insulator. If you want to enjoy RVing in the winter wonderland, look for an RV that has dual-pain windows for more warmth. Some RVs, like this Georgetown XL 369DS Class A motorhome, also come equipped with upgraded window treatments that help to keep even more heat from escaping.
Fireplace with Heater
This isn't a necessity, but boy is it nice to curl up in front of a warm fireplace after outdoor winter activities. RVs with a fireplace, like this Berkshire XL 40B Class A motorhome, are the ultimate home-away-from-home during the winter months. While some fireplaces are just for show and they only produce a pretty flame, others are equipped with heaters that help to warm the RV's interior. The heaters come in different BTUs, so ask your RV salesperson about the specific BTUs if you want a fireplace for heat.