Your coach also has a lot more insulation in the walls when compared to the cloth walls of a pop up

Additionally, the heat pump option units are not installed on a pop up, only the heat strip models. I don't know the exact reason for that one, but a couple of possibilities are additional weight (pop ups have weight limited roofs) and/or the fact that the heat pump units may only be available in a ducted unit. Interesting to know the fact about decreasing efficiency, I'll have to b=remember that when we look for a coach.
Now to the original question, at least a 13,500 or even a 15,000 unit on a pop up. Additionally, look into
Popupgizmos solar bunk end covers to help keep it cooler in the Florida sun.