We have a 25 gallon tank that holds 20 gallons when full. We use it for heat, the fridge, and hot water (2X per day). Daytime temps here are in the 40s, nighttime temps are in the low 30s. We set the temp for 64 during the day and 62 at night. We have one sunny day every three days or so. Our rig is 34 feet long with two slides. At night we keep the shades down to help keep the heat in. The 20 gallons lasts 14 days.
If we set our thermostat at 70, chances are it would last half that. But we would be able to sit around in shorts and a t-shirt. As it is, we wear sweats and are very comfortable.