I've had it into the shop to be checked several times, they tell me it is just the small a/c put in at the factory.It does get colder than 20 degrees below the outside temp.When it is 100 outside, it will not cold below 80 inside.Can anyone help?
l should have said that it does NOT get colder than 20 degrees below outside temp.

How far r u driving it on the 100 degree days. If u r only driving it a few minutes the AC wo not have enough time to cool off the inside. Durango is do have a lot of space in them so they will be hard to cool off.
If u have rear AC run it at the same time.

If the car has been sitting in the Sun, roll down all the windows when u first start driving for about a minute. This will get all the 140 degree air out of the vehicle & replaced with only the 100 degree outside air. Then turn the air on Max AC & make sure if it has a recirculate button that it is also on.
If u do not want to run with the windows down, drive on regular AC for about 5 minutes before u turn it to max AC. Regular AC is cooling off 100 degree outside air. Max AC is cooling of 140 degree inside air.

Tinted windows will also help. Even with ur AC running all the glass in that vehicle on a hot day will make it even more difficult to cool off than a lot if not most vehicles.

Chrysler generally makes the best AC system in my opinion. You're just fighting to many negative affects.

Tint the windows since the Suv is all windows>

no that is wrong, all Chrysler air conditioners will cool a lot better than that, l own a stratus that will get down to 58 degrees in 95 degree weather & it has almost the same system yours has,their missing something,it suppose to cool a lot better than that,get another opinion on it,one thing that was mentioned in another answer also tinting ur windows will help lower it about 15 degrees,but u should still get good cool air form it,what it is they just do not know whats wrong with it,and this is their way of getting out of repairing it,you may have a blend door not working right on it that will cause it not to cool right,good luck.