Just finished a trip with our 31' on an E-450 chassis Arkansas to California through New Mexico, Arizona, Needles, CA. Temperatures were near 110 in Needles. Dash Air worked great until Sun started coming in non-tinted windows. Fired up generator and ducted house air. Cab and living area stayed reasonably cool. Rear bedroom - a heat sink. Oscillating fan running trying to pump cool air into the bedroom helped.
When we got home, I purchased a Coleman Mach 8 9.2k BTU AC and added to the bedroom. A world of difference in cool down time. I prefer to sleep cool.
Smaller RVs probably do not need two AC units.