Hi,
There are a few rv'ers who have enough solar to run an electric heater for long. Wind is inconsistent and fairly noisy.
The advantage of running electric heaters is that often electricity is included in the campground fee. Therefore, in some senses, it is free. The costs for propane come out of your wallet over and above camping fees.
It is lots cheaper to run the furnace than to run a generator to operate an electric heater. However, if the generator is being run to recharge a battery bank, it is not a bad idea to run an electric heater at the same time, if the generator is big enough to support both.
When I was a weekend warrior I could manage one night of electric heat before I needed to plug in to recharge. Now that I'm full time I tend to only use electric heat if I have access to shore power.