This is a popular misconception. Heat pumps deliver more BTU's of heat than ANY type of electrical resistance heating. If one of the interior portable heat pump units is used inside the RV, you also do not have to worry about the outside temperature being too low, as you would with your exterior roof-top heat pump.
I may be a little confused about this statement but, the heat must still come from outside. Heat pumps don't create heat, they extract it from one area and move it to another. They don't extract heat from inside the RV and move it to the same area. The ones I looked at online seem very space intensive and the window closure for the duct didn't look very RV friendly. Other than that, they can deliver more heat energy than they consume down to a certain temperature.