I think you're greatly underestimating the heating costs. RVs are incredibly inefficient when it comes to insulation.
In fahrenheit, with 50 degree days and 30 degree nights- I'll go through a 30 lb. container every 2-3 days. But, we like it warm during the day at like 70-72. At night, we'll set it for mid-60s.
Most people that "winter" in their RVs will have a 100 lb. tank brought in and have a contract for a propane supply company come refill it.
If you're going to use electric for much, make sure you add a meter. I've read about quite a few disputes between family + friends who put an RV on land and were surprised by the added cost.