Yes as mentioned above, I would get out my fresh water hose on a 'warm' day and fill the tank once every two weeks. I could drain on a warm day as well. Because I was full timing, I would pick the warmest days of the week to drain and fill the tanks.
When I was living in San Diego CA, the fresh water supply was only 55F, and way to cold to take a long shower. By using a L-5 tankless portable water heater, I could heat the incoming fresh water to about 80F by filling at around 7 liters per minute (it is rated to warm the water by about 50F at 5 liters per minute hence the L5 name). The now heated to 80F tank will give a really long shower time, and that was great.
Warming the water with a 37,000 Btu L5 might help prevent the fresh water tank from freezing, so will putting a electric heater with thermostat near it. I had one set to low heat output - 800 watts, and set the thermostat to about 55F. This also warmed the compartment near the grey and black tanks, and kept my bedroom above the tanks warmed to about 55 all winter when I lived in Portland OR.
The slightly larger L-10 portable heater (also at Amazon.com) can heat more water with it's 55,000 Btu input, and also have a higher temp output at 1.5 GPM due to it's larger heat input. You probably have near freezing incoming water at your campground? Your 6 gallon hot water tank will not last very long under those conditions. If you could rig up a way to move the 140F water from the hot water tank to the fresh water tank, you could also warm that water to about 80F by moving say 12 gallons into a tank with 60 gallons in it.
You will use a LOT of propane in the winter in South Dakota. I hope that you are hooked up to a 100 pound or larger tank near the RV, it can be refilled by a delivery truck. They typically will rent them in your location. It also helps to be able to switch back to one of the factory installed tanks in a emergency, especially if you can not get a delivery during a snowstorm.
Once there is enough snow on the ground to pile it up around the base of the RV, you will be thankful that no more cold air will blow under the RV, and it will effectively insulate the basement a LOT.
I have some Reflectix insulation that I cut for every window in my RV. This helps prevent the heat from leaving the windows.
Good luck, I would not want to spend a winter up there!
Fred.
