Let's back up for a minute. You have an '89 with 75k on the odometer.
It's 24 years old. Just for good measure I would likely also power flush the cooling system and transmission (new screen as well). flush and replace the brake fluid. Service the differential. Replace the fuel filter. check hoses and belts for cracking and replace as needed. Skip any service the previous did within the past year.
How long has it sat undriven? What are the date codes on the tires? If the tires are 5 to 7 years old you should seriously consider replacing them, regardless of their outward appearance. Others here will tell you this as well.
If you haven't had this done then have an RV tech inspect the coach (and the roof). Have them exercise the appliances (fridge, furnace, water heater, AC, microwave), water pump, and test the converter - on propane, shore power, and generator. Cold weather camping is not the time to find out your new-to-you motorhome has no heat or hot water. The RV tech can tell you if the water lines run inside the coach (good to know for cold weather camping). Did the previous owner add heating pads to the grey and black water holding tanks? Does the water heater also have electric? Is the compartment where the water pump is located heated with ducted air from the furnace? Is the fresh water tank inboard or against an outside wall?
What are the cold weather camping recommendations of the park that you are going to? Where I am in Sequim, WA the park is responsible to the end of the water spigot and the you are responsible from the spigot to your RV. Does your park recommend heat tape on the water hose? It is mild enough where I am that I use a 10 foot water hose inside foam pipe insulation, taped with duct tape every foot or so. It can get down to about 28 F here. In this park we leave the valve on the grey water tank open and on the freezing nights we run the kitchen faucet at a slow trickle. Water running through the water hose keeps it from freezing here. Some folks here use heat tape. Ask the park's office what they recommend.
Get a space heater. If you rely solely on the furnace you go through a lot of propane very quickly. Ask the park if the pedestals are 50/30/20 amps or 50/30 amps or 30/20 amps. If they have the 50 or 20 in addition to the 30 amp service for your coach, ask the park office if there is an electrical surcharge for using the 50 amp or 20 amp outlets for a 2nd space heater. 20 amp = 110v outlets. Regardless, you should buy a 50 amp to 30 amp dogbone and a 30 amp to 20 amp dogbone. you can connect both dogbones together to go from 50 amp to 20 amp.
Get really warmer slippers to wear in the coach. A 1989 motorhome will likely have cold floors. Can you use the furnace while underway? Yes. I'm not a fan of the practice.
Edit: propane furnaces AND just plain breathing generates humidity. Your windows will get fogged with moisture. Moisture is not your MH's friend. Consider bringing along a dehumidifier. I have one and it's on the kitchen counter, draining into the sink - because the grey tank's valve is open and I don't feel like dealing with empty it.
My below Class C is not winterized. The holding tanks are not heated. I have a Stay-A-While adapter on the propane tank that lets me use an external propane tank. West of the Cascades it is relatively easy to winter camp. I have no winter camping experience east of the Cascades. I have no idea what you can expect.
Travel Inn Resort in Elma, WA January 19, 2012 (it completely melted within a week)
2005 Cruise America 28R (Four Winds 28R) on a 2004 Ford E450 SD 6.8L V10 4R100
2009 smart fortwo Passion with Roadmaster "Falcon 2" towbar & tail light kit -
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