Generally 3500 watts is enough. Just depends on what you want to run on your trailer.
Look deeper into the ratings of the generator. Is the 3500 watt continuous or intermittent duty?
It should be sufficient to run the air conditioner, lights and TV. It may or may not run the microwave and air conditioner at the same time.
I use a Yamaha 3000 boost inverter generator and it will not run the microwave and AC at the same time. But it has generally been sufficient for my use.
You may get better targeted responses by posting the make/model of the generator you're considering. Additionally for camping use around other campers you want to check into the noise rating of the generator.