The beauty of mini-split (actually, most inverter air conditioners) is that they become more energy efficient at partial capacity because they run slower to match reduced cooling requirements.
However, at 120 F outside, the A/C is going to run 100% capacity and the energy efficiency may not be as great as one expects. If designing for 120 F, which is considered desert conditions, you practically can’t add too much cooling capacity.
Note that at desert conditions, air conditioners require significantly more power and “may” deliver significantly less cooling depending on inside temperature. To achieve 80 F inside the motorhome while it’s 120 F outside takes a lot of rated capacity.
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