Julie, welcome to the forum. Lots of good information here.
So you are saying that if it is under 100* the AC works fine, but if it goes over it will not start up? Or does it run and just not keep it cool enough. These things will normally keep your rig 10-15 degrees below the outside temp.
We could use a little more information. Like what rig do you have, as in what size AC and does it have a digital thermostat.
Peace
Dave