I can only imagine what is happening in the interior of a motorhome setting around in the sun when not being used. 120 degrees on up interior temperatures would have to be ruining a lot of stuff inside both it's structure and what you have in it.
Considering what one pays for RVs, we keep the A/C turned ON (a high degrees setting) during the summer when sitting in the back yard ... along with a thermostatically controlled roof vent fan. In the winter we keep an electric heater set to about 50 degrees plugged in and turned ON all the time inside the RV to prevent mold/mildew formation and frozen water systems.
Penny-wise and pound-foolish usually winds up being foolish. ;)