We have a 37 foot 5th with one super slide (4 X 14 feet) and a slide in the bedroom. Total square footage is 364 feet. We have one 15000 btu ducted air conditioner with a ceiling fan in the living area. All windows are factory tinted and are equipped with outside awnings and metal mini blinds inside.
With the patio awning and window awnings deployed, the mini blinds turned to block the sun but still allowing us to see out, the A/C will keep us comfortable up to 100 degrees outside temp. At about 85 degrees outside temp we begin using portable fans to direct the cooler air into warm areas of the 5th wheel. Over 100 the fans are pointed at wherever we are seated.
We were still comfortable while seated using the A/C and fans at 105 degrees outside temp with 60% humidity. That's the hottest we have been in. It was a little warm to move around in so we watched movies until it cooled down once the sun set.
In higher temps we run the A/C 24 hours a day to help keep everything as cold as possible. I have also found that if I set the thermostat at 55 degrees instead of the coldest (50 degrees) the A/C seems to work better at providing the maximum cold. I'm guessing at 50 degree setting and humidity there is some icing which prevents maximum cold.