Our '66 TradeWind seems comfortable with 2 ceramic heaters down to around 20 degrees if the wind isn't more than 10 MPH or so. Windier or much colder and you will be cold. The rear area needs to be sealed off from air infiltration somehow and for more than 1 night of this temperature range I would recommend draining the plumbing and using more primitive methods. The water pump doesn't like ice chunks or slushy water.
If you opt to run a propane furnace, be prepared for a lot of usage in the winter. We can use up a 30 lb tank of propane in 2 days if the temp drops below 20.
Lou Axt, Jr.
1969 Chevy C10