That really depends on the climate and model of your RV. If it's super hot out, then cranking it up to the highest setting will help keep you cool, but if it's milder, then a lower setting might be more energy efficient. Experiment with different settings to see what works best for you.
Keep in mind that most RV air conditioners don't strive to attain a specific temperature, but instead, they usually cool the interior of the camper by 20-30 degrees from the outside temperature. So if it's 110°F outside and your rooftop air conditioner is able to drop the temperature by 30°F, then the coolest you'll see your camper is around 80°F.