We lived in a 30' 5th wheel with a 13' slide for one winter and used a 30 lb bottle every 3 days when it was cold and the wind blowing. Biggest issue is the wind. It may be calm and 20 degrees no big deal but 30-40 degrees and 35-40 mph constant winds keeps the heater running most of the time.
And of course how well the unit is insulated/double pane windows/artic pak/etc can determine fuel usage.
Our unit had a R7 top/bottom and sides but I did install mobile home skirting and 1/8' lexan as storm windows on the inside. Both helped a bunch.