I can tell you that mine with no heat sources inside (like people) and a window cracked, uses about 13 W/degree F. That is the temperature difference between inside and outside. 500 W (x 3.4 = 1700 BTU/hr) would maintain ~38 F temperature difference for instance. I measured this with an electric heater, run for 3 days so it was saturated. It's not a poptop, has 1.5-2" thick foam walls, and 3 small windows, 2 of them single pane glass. I did not have them covered with insulation like I normally do if it's cold.
There are so many variables like the amount of wind, radiation to space at night, the amount of infiltration, sun, how many people and dogs, etc. Most heaters made for campers have plenty of output, but it's good to have an idea how much fuel you'll use.
