Generator??
The first question to be answered is: where will you carry (and run) the generator? If you expect to install it in, and run it from, a compartment in the trailer, then the size of that compartment is very key in choosing the generator. Generally, ONAN makes most of the built-in generators, although I think Generac makes one as well.
If you are going to carry the generator in a compartment in the trailer (or in the back of your truck) and remove it during use, then you can get anything you like that will fit. Honda 'inverter' models are very popular due to the high efficiency, low noise and very clean power. Yamaha has a few models which may compete with these.
The absolute minimum, for 1, small, A/C would be about 3000 watts. You may need to install a 'hard start' (or 'easy start'

) kit to increase the reliability of this setup. 4000 watts would be better. 2 Honda EU2000 hooked together with a synch cord is a popular, easy to carry, setup, but also easy to steal due to the small size and light weight of each unit.
Keep in mind that if your generator is loud or obnoxious, you may be limited when and where you can run it.
If you have a bigger A/C or 2 A/C units, you will need more power, probably in the 6000 to 7500 watt range. 2 EU3000 hooked with a synch cord may do the job, but boy will it weigh and take up space.
If your generator is built into the trailer, you may want to consider running it off of propane to easy your fuel management. On the other hand, propane tends to be less efficient, so you'll need a bigger generator than you would with gasoline. Also, altitude decreases the efficiency of generators, so it you spend a lot of time at height, you will want to have a bigger generator (or at least have it adjusted for the height).