We did a Late May trip to Lancaster, PA (Sight & Sound Theater) in our pup with no heater. Unseasonal cold weather was 50ish during the day, low 30's when the sun went down. It was to cold to sit around the fire...but a small heater (extension run below canvas out to the pole) kept us snug inside. We spent a lot of time watching netflix on a laptop that week.
There is a show on t.v. called "life Below Zero". One guy "camps" 65 miles below the artic circle in a framed canvas tent with a small wood burner. Many of the people on that show frequently camp out in tents with a small heater in sub-zero temps.
Point being....once the space warms up, it is just a matter of expending fuel to keep it warm. Propane gets expensive, but if electric is available a small heater can keep you comfortable at 70 degrees. It will run the entire time, but you paid for the electric site...right.