Most Rooftop RV Air Conditioners need between 2500 and 3500 watts to start them. Many will run on less amperage but starting them requires a lot more power. This causes a problem for many of the 2000 watt generators to handle so you will probably want to go to 3000+ watts for your generator.
Another problem with needing 2500+ watts in your generator is the weight. Most of the 3000 watt standard utility generators weigh from 125 to 175 pounds. This does not include fuel. This is often too heavy and too clumsy to be handled by one person. The last thing you want to do while camping is injure or throw out your back.
If you haul your travel trailer with a pickup, a pretty good option is to leave the generator in the back of the pickup. Locking it to the bed is also a pretty good idea.
Two Small Generators
Another great option for running a rooftop air conditioner on a RV is two use two small generators that can be hooked together to work in tandem. There are a couple of advantages to this option, the biggest advantage is you can use a couple of 50 pound generators instead of one 100 pound generator.
Suggestion: Buy your two generators and the connecting system all at one time and make sure they all work together. Getting one generator one year and then planning on adding another generator and the connecting gear the next year might not be a good idea. As companies come out with new models there may be compatibility issues.
Both Honda and Yamaha make smaller generator that can be hooked together to work as a much larger generator. Be sure to check when you are purchasing the generators that there is a connection system for the generator you are purchasing.